Well you've figured out about light traveling underwater - your strobe will not light beyond 5'. You are also right about letting them come to you. You actually got down pretty good - if you had dropped down another foot or two when you shot at more of an upward angle the shark would have been against the water background instead of blending in with the reef.
Your composition is good – vertical lines usually look best and you got the whole guy – even gave him some room to swim in the picture (vs. having his nose against the edge).
Does your housing have a red filter that can be moved in front of the lens? If so, try using that with some available light and see what happens. Keep up the good work.
Remember the mantra: Get down, get close, shoot up.