Getting a dive computer is a major investment. There are different models of computers and prices vary. Some models come with software that allows you to download dive data to your PC. This is a nice bonus for divers who want to track their dives in a more permanent way.
The best freediving computer will be easy to read and measure time accurately. It should have a large display and backlight. This is important for visibility in low light.
A freediving computer should also have an alarm function. An alarm will tell you when you are getting too deep or ascending too fast. It will also help you signal where you are in the dive. You can also set an alarm to trigger at a specific time.
Another feature to look for in a freediving computer is a dive time alarm. This tells you when you have been underwater for x amount of time.
A dive computer will also tell you when you are ascending too fast. Most have an audible alarm that will start beeping underwater. These are very helpful in keeping you focused.
You also want a computer that allows you to measure your breathing rate. Some air integrated models will also calculate your time to decompress and your oxygen consumption. This is useful for divers who want to know how long their air supply will last.
Another feature to look for in scuba models is apnea mode. This is helpful for divers who are using nitrox. The computer will calculate how much nitrogen you are breathing when the current is behind you.