Dive Watches are more than just an accessory. There are many different kinds of watches meant to carry out different functions and some of them can do more than just telling the time. Dive watches may appear to be somewhat complex looking, but they provide divers a very important function: keeping track of how much air they have in their tank.
Seiko and Citizen both were brands that honored on SportDiver's list of the top watches in 2015. Both brands are known for exceptional dive watches and the Seiko Prospex Marine Master and the Citizen Promaster Diver are two particular models that stood out amongst the rest.
When buying a dive watch, some important features to look out for would be how the watch is powered, the bezel, and if it is ISO compliant (certified as a dive watch).
A watch that is movement powered like the Seiko Automatic is fine if you wear it often. However, if you aim to only wear it on certain occasions than extra measures should be taken to make sure it is operational before diving. The Citizen Eco-Drive's light powered functionality might be more appealing to those who seek to purchase a dive watch that is convenient and easy to maintain
Second, the bezel. The Seiko Sumo has a one-directional bezel with a luminous PiP at 12:00. This may be something to consider if you want the mark to be more noticeable. The bezels can count up or down, though most prefer elapsed time. Aside from aesthetics, you want to make sure the bezel works as that is very important for a diver.
Finally, ISO compliant is the certification that it can go 100meters or 330ft at the minimum. After that, you can have a large faced watch like the Seiko Prospex, or the more stylish design of the Citizen Eco-Drive. The face, band and size are all preferences after functionality.