I just installed 3 subs in the Donzi. 2 of them are firing forward and mounted under the rear seat. The other is in the cabin, down low firing rearward. Do I need to wire the cabin sub out of phase with the cockpit subs?

They are JL Audio 10IB (Btw...these things are AMAZING, I walked outside the shop, a 40x80 morton, and it sounded like the siding was about to fall off!)

Maybe, but the only way to know for sure is to try it both ways and pick what sounds best.

I hope you are talking the 2 vs 1, you don't want to fire the ones sharing an enclosure out of phase

I would have to guess that you want them all firing together, but I agree with about, worth a shot to reverse the polarity on the cabin sub and see if you like it.

You may find the acoustical resonance outside of the boat vs in the boat has different demands. The reversed sub may sound better in the boat, but worse away from it.....or vise versa.

You're siding is coming off, with JL's you're on track. On the water should be awesome