Dive Computers
Although divers once hit the water with no computers for generations, there is no doubt that the convenience of dive computers has made these handy devices a permanent part of today’s diving experience. While you don’t strictly need a diving computer to dive, a computer can definitely make diving more convenient, enjoyable, and safer as well. For these reasons alone, you will want to invest in a diving computer. Diving computers offer many features, and you will want to look for the features that best meet your needs:
1) Diving calculations. Diving computers do many of the calculations you need. For example, a diving computer can tell you when you need to make safety or decompression stops. Many diving computers can tell you when you can safely travel by plane after a dive. Best of all, diving computers adjust automatically and based on your depth and underwater time, are constantly recalculating your predicted nitrogen level. Advanced  computers can calculate O2 levels as well, so you can always dive safe. If your bottom time is reaching unsafe limits, your nitrogen level rises, or if you are ascending too fast, modern dive computer will let you know about this. If you miss a safety or decompression stop, your diving computer will note this and recalculate your next stop based on this data.

2) Depth gauge. Diving computers note your depth, the maximum depth reached during your dive, and the time you have at the current depth before you need decompression stops during your ascent. A depth gauge on your computer is more useful than your traditional gauge, since your computer does not require you to constantly keep checking – the computer does all the work for you. You can also set up your dive limits yourself and yourdive computer will signal during the dive if you have reached you maximum depth.
3) Air-integrated dive computer features. Air-integrated dive computers allow you to connect the computer with your tank via an air hose so that the computer can display PSI, air time remaining, and other air information. With an air-integrated dive computer, you may not need your pressure gauge. On the other hand, if you already have a pressure gauge you might not want to spend extra cash on a more expensive air-integrated dive computer. Keep in mind, too, that if your air-integrated computer experiences any type of error, glitch or simply freezes (all very rare occurrences) during your dive, it is possible to loose some information or data. For example, you might not be able to tell how much air is left in your tank.
4) Nitrox/Air dive computers. You need to decide whether you will be using Nitrox at some stage. If you currently use or might use Nitrox, you will need the more expensive Nitrox-compatible computer, which lets you program the percent of oxygen in your Nitrox mix.  All Nitrox-compatible computers can also be used for regular air-dives. You typically have two options: a) Just set up 21% of oxygen for your gas mixture; or b) use the dual Nitrox or Air modes, available on most Nitrox compatible computers.

5) Computer compatibility. Some diving computers give you the option to connect to a PC and use manufacturer's software to download the details of your every dive onto your home computer.  Dive computer software can create dive graphical charts based on your depth and time for each dive. More advanced computers will be able to provide even more detailed information about your nitrogen and oxygen levels, ascent rate violations, and even air consumption. All this information can be stored permanently on your computer, allowing you to keep more detailed logs and helping you to analyze and improve your dives. 
6) Memory. Diving computers come with specific memory storage. Some only record a few dives, while some record several. If you like longer diving vacations, you may need a diving computer with a larger memory or one that is PC-compatible if you want to save the details of each dive. Most of today's dive computers can store an average of ten dives.

7) Lighting. If at all possible, choose a diving computer with backlighting, which will let you see your display in different lighting conditions.
8) Mounting. Diving computers are either boot mounted or wrist mounted (like a wristwatch). Deciding on which style you prefer depends on your gear and what you feel most comfortable with.
9) Auditory signals. Some diving computers include beeps or buzzers that warn a diver about exceeded oxygen toxicity limits, too-fast ascent rates, exceeded maximum operation depths, missed decompression stops, and other dangers. If possible, it is a good idea to get diving computers with these signals, as it keeps you safer without you having to constantly keep an eye on your computer.
10) Activation. Some dive computers are activated when you hit a button and others activate automatically when submerged in water. If you may forget about turning your computer on and off, it may be safer to choose water-activated diving computers.

11)  Navigation features. Many diving computers today come with a compass which can be used easily (even on unleveled surfaces) and which can replace your old compass. Some diving computers even come with a Global Positioning System (GPS), which can help you track exactly where you are.
Diving computers can be as simple or as fancy as you – and your pocketbook – can stand. In general, though, you will want to balance functionality with price. Look for the features you need rather than spending more for features that are simply “cool” without being much use to you.