We had to replace ours as well. The housing cracked and there was no way to keep the bulb in place after the old one crashed and shattered on deck. We had no luck at all finding USA distribution for that particular AquaSignal model light fixture. We managed to source the LED version of the entire fixture on eBay through a supplier in Lithuania, of all places.
I added some silicone to help secure the bulb in place.
(Boat’s in Charleston, we are in Antigua wrapping up a delivery)