PCS ARE TAKING OVER THE WORLD. Just about everyone I know either owns or has access to a personal computer of some sort, and those of us who use one regularly soon come to depend on it.
     We use them to write letters, send and receive e-mail and faxes, browse the web and plan dives using decompression-planning software. Many hardboat skippers have PCs by the helm, displaying electronic charts, plotting their position from a GPS interface and even steering the boat's auto-pilot.
     A PC is a remarkable general-purpose tool which can be used to fulfil many special-purpose roles. How did we ever cope without it?
     Before PCs became cheap and readily available, we used special-purpose tools. To send and receive faxes, we had fax machines. To write letters, we had word-processors, typewriters or even a simple pen and paper. Rather than search the divernet.com website, we pawed through back copies of Diver to find that vaguely remembered article about the wreck we would be diving at the weekend.
     The move to general-purpose computers open to many applications is part of a cycle that has been repeated many times.
     The earliest computers were special-purpose machines built for applications such as plotting gun trajectories, coding messages and breaking codes. Breaking codes was a landmark. That's when computers first began to evolve into more general-purpose, room-sized machines.
     Even so, these were still sold as tools for specific applications such as running a business's accounts or payroll, with the hardware and software bundled together by the computer manufacturer.
     A far cry from today, when you buy a PC from any one of hundreds of suppliers, an operating system from another (but usually Microsoft), and applications from thousands of suppliers.
     As history has repeated itself many times, imagine the future if this pattern were repeated with dive computers.
     Our current dive computers are comparatively primitive. The more sophisticated are still only at the level of a primitive word-processor compared to a basic typewriter - fine for writing letters, but only if all you want to do is write letters using the same limited process conceived by the typewriter's inventors.
     Dive computers are overdue for the next evolutionary step, from that of special-purpose deco calculator to general-purpose underwater computer, able to be loaded with applications for decompression and other useful things.
     This could come about through existing suppliers opening up their hardware to third-party software applications, but why waste time with hardware specific to diving?
     Palm-sized PCs or personal digital assistants (PDAs) are compact and readily available. All we need is an underwater housing and a pressure-sensor to measure depth. We would then have a mature platform with all the economies of mass-market hardware, ready for some enterprising diver to write a dive-computer application.
     Using a palm computer as the basis for a dive computer also has the advantage of standardised interfaces. You could start with a basic computer and housing to run deco software, then grow the system with more advanced hardware and software to match the way your diving develops.
     Some obvious ideas such as cylinder-pressure display and oxygen-monitoring for a rebreather have already been integrated into more advanced dive computers, but why stop there? The computer could control the rebreather.
     Add an interface to a positioning system for navigation. Link this to the equivalent of a chart-plotter and you could follow a digital Wreck Tour downloaded from divernet.com.
     Take pictures with a digital camera and transfer them to the computer to be stored against their location on the wreck.
     Back on the boat, plug into a mobile phone and transfer the dive to your website, or even transfer it as it happens, using an interface to the underwater mobile phone recently developed by a French company. Deco-stop boredom would be relieved by Laura Croft and the latest underwater Tomb Raider.
     Some will argue that the convenience and economy of today's special-purpose dive computers will ensure their survival, and that general-purpose underwater PCs could only ever be a niche market.
     Then again, some people used to say that typewriters would never be replaced by PCs running word-processor software. Many PC-owners only ever use their machines to write letters, but how many would go back to the typewriter?
     Some of my applications for an underwater computer may seem far fetched, but they are not revolutionary. All are based on current technology.
     All we need is that housing for a palm computer, and even if many of us only ever used the basic dive-computer software, the underwater equivalent of the typewriter could soon be obsolete.