Of course the answers to all these questions are value judgements and carry a price tag! Today, all dive computers perform critical basic functions:
Time of and length of your time underwater;
Length of your surface interval;
Depth and warnings if you are approaching decompression limits;
Self-recorded oxygen percentages in a Nitrox mix; and
Safety stop displays.
At the Less than $400 level, your dive computer will perform all the basic functions but will not tell time as a clock.
At the $400 to $500 level, your dive computer will tell time and record additional indicators like a deep stop should you desire to create one.
At the $500 to $700 level, you can expect the computer to be able to perform additional functions - count swim laps, or graph your dive, or tell direction with a digital compass.
Above $700, you can get an air-integrated dive computer. That means the digital display will tell you, in real time, how much air you have left in your tank. Remaining air is recorded as pounds per square inch (PSI) on this side of the pond!
To know your remaining air, your computer must be linked to your tank through either a high-pressure hose connecting the computer to the regulator or through a transmitter which is attached to your regulator. Dive computers that display information in a console are linked by hose. Unless these computers sport an eLed display, they are usually less than $1,000.
Dive computers that rely on transmitted data are usually wrist computers. Most are at or just over $1,000.
Beyond the $1,000, you can have your pick of bells and whistles, (although we haven't found one yet that will make coffee or walk the dog!) Dive computers priced at more than $1,000 can include:
Air remaining (air-integrated) in one or more tanks;
eLed displays (neon lights on a bed of black velvet);
Real-time graphics illustrating your dive profile;
Rapid-response digital compasses that tolerate tilt; and
Early-warning signals of nitrogen poisoning or oxygen toxicity.