As the motorboat whisks us further out from Key Largo, I can see through the morning haze the outline of an anchored barge.
No ordinary barge this, four miles off the Florida coast, but the life-support system for the Aquarius undersea habitat.
I am escorted to the mission control centre, an area filled with sensors and monitors. Having dreamt of being a true aquanaut when I was a young boy, it now seems that I am finally entering the world of Jules Verne.
Living 15m beneath us are six divers, scientists who stay submerged for ten days at a time, in a cylinder anchored to the seabed.
Aquarius is currently the only undersea habitat devoted to science. The National Oceanographic Atmosphere Administration (NOAA) in America has run other projects over the past 20 years - not always without incident, but its experiences have helped it to evolve its safety procedures to a fine pitch. The Aquarius habitat has an excellent track record of 21 missions off Key Largo, and has revolutionised coral-reef research.
Permission to visit the aquanauts granted, I am briefed by the support team before entering the water.
As I approach the seabed, the habitat looks far bigger than I had expected. It is in fact 13m long. Schools of sergeant-majors and grunts hug the coral-encrusted superstructure. I swim along it, examining the various experiment posts.
In the wet porch, where water meets air, I am greeted by the smiling face of one of the aquanauts, Dione Swanson. I start grinning too when I hear his high-pitched voice, then realise, as I speak, that my voice sounds equally like that of a cartoon character.
This, I am to learn, is caused by the high pressure acting on our vocal chords.
The other aquanauts emerge to welcome the stranger to their undersea home. I am asked to remove my equipment and take a shower to decontaminate me before entering the living quarters. These feel comfortable and homely, far from the damp, claustrophobic environment I had expected. The habitat is fully air-conditioned, with a decor of stainless steel fittings and blue carpeting, and is bathed in soft blue light.
"All we usually get is support divers to see us. It's a nice change to see somebody new," says Dr Peter Edmunds, the head of the team.
It is he who explains about the Mickey Mouse voices. "You're lucky," Dione Swanson tells me. "If you stay as long as us your taste buds change. After a couple of days nothing tastes any good." He turns to the others for support. "Doesn't the food taste so bland?"
I had been told to bring magazines for the aquanauts. As I reach into my camera bag, I feel I am being watched by a pack of starving dogs - starving for intellectual stimulation! I hesitate. Should I perhaps use the mags as incentives in meeting my photographic needs?
We sit around the table, drink coffee, eat biscuits and watch the sea life go by outside the portholes. David Carlon gives me the guided tour - the submarine-style toilet; a bench of computers; digital displays for marine sensors. At the far end of the habitat is a cramped area where six bunks hug the hull wall.
"The centre section is where we relax, eat meals, discuss plans," he says. "We are monitored throughout the mission, electronically in the habitat and visually by support divers on the reef.
"This is necessary for safety reasons but it adds stress to living underwater in a hostile environ-ment. Everywhere we go there are cameras monitoring our movements at the surface. There are microphones and speakers allowing conversations to be listened to, and commented on, by members of the support staff on the surface.
"The only place we can get a bit of piece and quiet is on the toilet!"
The PA blares. "You have five minutes to be on site, why are you still drinking?" yells a member of the topside team.
The aquanauts make their way hastily to the wet porch to don their wetsuits. They snorkel to the bottle racks and fit their double tanks before entering the air-filled gazebo.
I catch them before they leave for the reef, to ask how long they will be gone.
"Probably four hours," I am told. "We could go out for up to ten hours, but we all need a break."
The limiting factor is the cold, even in the temperate water of the Keys. "After a four-hour dip the skin does not look too good," an aquanaut tells me. "We all have problems with our feet, as they get sore after such long submersions. When I get back I'll show you all the plasters on my toes!"
But as a visitor at this depth I know I am limited on bottom time and will have to forego that pleasure. I have only a short time to spend with the scientists.
"Must go now! Diver leaving the surface!" shouts Dione. The type of research being carried out by the team could not be done using conventional scuba diving. The divers have to be put into saturation.
They descend to their working depth for about 24 hours, after which point their tissues are saturated with dissolved nitrogen. They can then make excursions to required depths for effectively unlimited bottom time.
I follow the scientists as they swim along the lines laid from the habitat across the reef to various workstations. At the ends of the lines are air-filled domes, into which the scientists can stick their heads, talk science, eat fruit and sweets. Here they can also take on fresh sets of double tanks and continue their dive excursions as far down as 30m.
I leave them to their work, and am told it is time to make my way back to the surface. I have been down for almost two hours. Back in the hot sunlight I enjoy a hearty barbecue with the surface team. They show me around the barge. In a container on the back deck a plush double-lock recompression chamber is fitted, to treat aquanauts and support divers. It has never been used, but is kept in standby mode.
I am introduced to Dr Steven Miller, the Science Director of the National Undersea Research Centre of the University of North Carolina. He has been on numerous missions since the start of the Aquarius programme.
"The purpose of this mission is to record the growth of corals, by photographing and measuring them," he says. "From this data we can decide whether the juvenile corals are settling into their new environment."
Dr Edmunds and his team are apparently coming up with many unexpected findings. One is that the large star corals Montastraea cavernosa account for more than 15 per cent of the juvenile corals in some locations, and yet are rarely seen as "adults".
"Other experiments include the measurement of animals' intake of nutrients using small chambers containing coral juveniles. The aquanauts collect regular samples of the water for analysis in their laboratory."
I ask how the habitat's life-support system works. "We have made Aquarius into a semi-closed rebreather," says Dr Miller. "We have a desired flow of gas going into the habitat constantly, and it passes through a carbon dioxide chemical scrubber. The gas is monitored in the habitat and on the surface, through special sensors. This system seems to be the most cost-effective for such a long duration."
Although the scientists work in warm tropical water, the effects of fatigue and nitrogen narcosis seem to be intensified. I had noticed the way the team laughed about almost anything. This, I was told, extended to outbreaks of "the funk", an unpleasant rash caused by dampness and abrasions.
This behaviour worries the surface team; the aquanauts have been known to run around erratically for up to ten minutes without answering calls from above. They had been warned about such behaviour before the mission, as it is common in divers whose tissues are saturated with nitrogen.
After ten days of rashes, cuts, abrasions and swollen fingers, the thought of a dry existence becomes an incredible luxury for the aquanauts. It becomes reality when the decompression phase of the dive starts.
This takes 16 hours, as they undergo a pressure change effected inside the habitat. During this time they go through a well-practised deco schedule run by the topside team before swimming to the surface and basking in the sun.
Compared to other scientific projects, the annual operating costs of about $1.2 million for the Aquarius project are modest. On a daily basis it costs less than an oceanographic cruise to run; the cost of a single Space Shuttle mission would fund it for 500 years.
However, with the US federal budget chopped, Aquarius has been on the block ever since it began. Let's hope it survives, allowing us to continue finding out more about the incredible reef ecosystem.