YES! A focus light is a very valuable tool whether you have a point & shoot or D-SLR. A focus light will assist your camera in seeing the subject more clearly and thus improve focus speed and clarity. Usually most helpful when shooting into a darker space like a ledge or hole or of course at night.
F The simplest is an attached flashlight. Either to your housing or strobe arm. Not the best, but the cheapest. Most UW handheld lights are heavy and underpowered for best results in this application. Next might be a built in focus light into your strobe. This is how I started, and it is a good solution. However the limitation is that when you want to aim your strobe one way and the subject is in another direction you limit your artistic abilities. My new favorite is a dedicated and specialized tool just for the job. There are now focus lights with features that make them more than a flashlight. My favorite is from Hartenburger.
They have a couple options for you. These lights have variable intensities, mounting options, and charging capabilities. They can be a hand held for models to hold, you to use traditionally, or mount them to the camera housing or arm for their intended use.
Consider a focus light as your next toy in the box.

For what it’s worth…
If you are considering entering or upgrading the underwater photo experience, here is my thoughts on what and where to buy.
First you really need a mentor, someone who’s photos you admire and can share this experience with. Check with you local dive center, or dive club for such a pal. I believe to be more successful at something, you are best served by aligning yourself with those who have achieved ahead of you.
In the D-SLR category, I shoot Nikon bodies and lenses. This is because my mentor does, and now so do all my friends. I am glad, even knowing now what I did not then, Nikon is versatile and easy to learn. After selling several brands of housing and seeing many more in use, I am very glad I shoot in a Sea and Sea housing. I know they are the most ergonomic, best built, complete underwater systems available. Yes you will pay for it, but the value is priceless. You could pay much more for a lesser product. I am however not a Sea and Sea lighting fan. Ikelite is still the hands down fav there. I have even had more than one warranty issue, I must say they are professional and fair. Great experience. Ikelite is innovative and quality built.

So the recommendation would be…D-SLR: A Nikon D80, D300, or D3X…the prices are lowest to highest in this range from $799-$4999. Here is one of the only times you will ever hear me say this, GO ONLINE…B&H Photo Online! Great supply, service, warranty experiences all around. Unless you or someone you know is going to St. Martin, like coming to see me in Saba maybe…or on a cruise. Bauchands is a similar place with lower prices, same U.S. product and warranties, and NO TAXES! Save baby!
So put this baby in a Sea&Sea Housing with Ikelite DS125 or DS200 strobes, Go with TTL Sync Cords, and really any arms are fine; bought at your local dive/photo retailer please.

As for Digital Point and Shoot options: Olympus or Cannon, the highest end P&S you can afford and that has a housing available…see www.digideep.com for that answer. Go with the brands choice of housing if a Sea&Sea is not available…not a fan of the Ikelite All-In-One option either. The strobe of choice is Inons highest end option with fiber optic TTL. Especially two of them!

Check out these links:
Sea & Sea

Ikelite Strobes

INON

For more specific recommendations email me, just click on contact above and fire me your questions. The list of  options is long, but I hope this starts you off on the right path to having great success.

For product reviews and news, check back here regularly. Much more opinions coming soon