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.