There are plenty of lost fishing nets, coral, ropes, etc. underwater to trip you up so you may want to consider carrying a diving knife. They are sold in many different shapes and sizes. A diving knife can help you get through those obstacles underwater. They are usually strapped onto your lower leg or upper arm with rubber straps and contained by a rubber loop.
You may also want to consider getting a depth gauge. A depth gauge is used to measure the depth of water you are in at the point when you look at it.
If you choose to take a watch underwater it obviously has to be waterproof and you will probably want one with a fairly large face that you are able to see clearly. It should have a long rubber strap so you can adjust it to fit tightly. Diving watches can also have a rotating bezel which will allow you to record elapsed time. This feature is very helpful.
A float or a reel may also be a smart decision to use when you are snorkeling. You should buy one that indicates you are underwater. It should have an "A" flag on the top which is a warning that there is a diver underwater there.