Since you have two of them, it's very easy to narrow it down.
I'd first swap the bulbs around. See if the problem follows the bulb or stays in that particular "position" in the hood. If it follows the bulb, you have a bad bulb. If it stays in place, read on.
Next, check the socket for corrosion. This is very common in saltwater. If it's corroded, clean out that corrosion. If not, read on.
Check all the connections between the hood and the ballast (if the ballast is remote). Make sure everything is clean, dry, and tight.
Again, if it's a remote ballast, flex the wires all the way down from the hood to the ballast with the lights running. If the lights flicker, you have bad wiring that's easily replaced.
If you can't get any results from the above stuff, then your are probably right. Most of the big aquatic houses (
*cough*marine depot*cough*) have ballsts. The trick is finding one that fits. The best and easiest thing to do is to take the fixture or ballast pack apart and find the manufacturer/model of the ballast. Most ballasts are made by a handful of companies (Pony, Icecap, Magnatek, Advance Transformer). Getting the model of the ballast and googling around will likely yeild a company that not only sells the exact ballast but sells them to ordinary idiots like you and I. :hilarious
If you fixture is a cheap overseas import (to put it delicately), you may be stuck having to make another manufacturer's ballast fit... that's when you enter "Drill-N-Save" mode and have to rig your fixture to fit the ballast.
If all this isn't your thing, most high end LFSs will either do this for you or will have guys working there that moonlight with this kind of thing and will do it for 20 bucks plus parts... etc. Good luck!