Another crazy idea I have used in the past.....
Several years ago I owned a Chris Craft Cavalier plywood cruiser. Being plywood, there were no duoble-walled hidden places to put speakers. Any hole you cut in the cabin either exposed the cockpit, the head, or the topsides.... Any hole you cut in the cokpit exposed the cabin, the head, or the topsides....
The typical solution in these boats is an enclosed speaker, boxy, usually white, which predictably faded to yellow. Not a good soluition for my aesthetic.
SO...
What I did was go out and buy some plastic hanging pots, 5" to 6" in diameter, some large-perf metal mesh, some of that window insulationg putty, (you could use butyl tape) and some silk ivy.
Using some very small speaker wire, I carefully twisted/braided the speaker wire into a 3-point hanging system for the pot, with one of the wire pairs actually being the real signal conductor. I then took a speaker that was smaller, with a round OD feature and "glued" it into place down in the pot. For speakers that have tabs, consider cutting a round ring that the speaker can screw down to, and then fasten the ring to the plastic pot With the metal mesh I cut a disk and glued it in place, then poked enough silk ivy into the mesh to make a convincing hanging plant.
I hung a pair of the plant speakers down in the cabin, to port and starboard. up in the cockpit, I installed another pair aft witn one on either side, under the wide gunwales...
The pot affected the sound some, and added some coloration... It was noticeable to me but not to most others. Cosmetically it just looked like my wife had aded a touch of decoration to the boat.....
You guys might find this, or ssomething similar might be the ideal solution. Who knows... Figured I would share.
