I have an underwater light in our canal at our place in Galveston. I can tell you that they really do attract fish, and quality fish at that. I've got one that has a weight and the bulb suspends just above the bottom. Have had no issues with marine buildup or bulb maintenance. Light comes on every night and burns off any algea. Did have a couple issues with the RFI plug tripping and the dawn/dusk sensor that triggers the light on and off, but those were not directly related to the light itself, and have not had any problems in over a year.
They do have drawbacks though, fish are very spooky in underwater lights. You can't hang out on the dock making a bunch of noise or fish will move out of the light. If you catch one or even get a bite, fish will scatter and you gotta give the light a rest for 15-20 min before they come back. I like to catch a fish, go up stairs and sip my beer for a few minutes, watch TV, come back down and they are back, catch another, rinse and repeat.
Fish won't be as spooky if the light is in deeper water in a main canal or cove. More wind/current/ripples to distract the fish from what is going on. We are in the back of small canal so don't have that luxury.
I've got the overhead penetrating lights also and I tend to like those more if I'm actually going to be "fishing" rather than "stalking". Better for making blind casts and not having to worry about scaring the fish off or anything. More consistent I would guess, especially if the fish are there.
Both are good lights, but just a different approach of how to fish in them IMO.
So basically what I guess I'm saying is get both types of lights ...haha gotta keep your options open.