I apologize for bump the old topic, but I have some interesting observations that I would like to share...
I had a similar situation as you that I made a low cost fix that I'm happy with in my 21' Crownline. I started with a JL 13w7 https://atfulldrive.com/best-car-subwoofer in a ported box in the engine compartment area on a JL 1000/. Loved the sound and output, but it would deplete the batteries quickly - like an hour, not much more. I have 3 for the amps and 1 for starting and the HU. I then added two JBL GTO 1214D ported to 30 something, each under the helm, powered by a single JL 500/. I made a remote toggle switch and gain dials for each so I can use one, both, or none. The ones in the helm help the midbass a lot while we are cruising along, the 13w7 brings the low bass and the tactile feel. When we are coved out, I turn off the w7 and run the JBL pair. We can set for about 8 hours at high volume before the voltage is too low for the amps.

In my pontoon boat, I use all Morel 6x9s ib, but am building a slim T48 horn using the Eminence 3015lf which will blow the GTOs away with less power. It's a huge box, but it's a pontoon so I have room plenty of room and can put it in the back with a cushion on top and use it as a seat.
There are a lot of useful videos on this subject on Youtube, I'll leave one here, Hope this helps someone. Good luck!