As you probably know water absorbs the colors as you go deep. Red goes first, blue is the last one.
If you are not using a strobe then you can forget from getting any colors unless you are taking photos at a depth less than 2-3 meters and it is a nice day without clouds and the sea is calm. Everything that can disturb the light on its way to the water will disturb your photos. Another thing you can do pretty well without a strobe is silhouettes.
Now you take a strobe with you: Same color absorption. So basically you can't shoot photos at distances more than 1.5-2 meters! It doesn't matter what strobe you have!
So getting as close as possible is the best thing one can do to get better colors. A famous UW photographer said that the best thing one can do to improve UW photography is to remove the water...Can't remove it? Minimize it (the amount of water between lens and subject).
Getting close and you don't get good colors? Hmmm...Perhaps your strobe is aiming somewhere else and not to your subject?
What sort of film are you using? I'd prefer to stick with the basic 100 ASA's. They have better color saturation than faster film (although all the film manufacturers will swear that their 800-1600 ASA film have brilliant colors you don't have to believe them). Don't worry, there's enough light down there for 100 ASA if you take a strobe, and without a strobe you'll be shooting at shallow water so no problem there neither. My prefered one is Fuji's superia 100 ASA. It's also a very cheap.
Last ammo in your arsenal can be "Photoshop". If you scan the photos you can revive the colors to some extent, reduce some of the blue, increase contrast etc.
