One amp to power all speakers?

[David Analog]

Any thoughts out there about the Alpine marine amps?

The Alpine PDX amplifiers are legit. Alpine was one of the pioneers in automotive fullrange Class D.
Eons ago, the home stereo division of Alpine was Luxman. Serious stuff.

[brain_rinse]

Had a pdx amp running my exile towers and was very happy with it. High quality amp for sure.

[Klipper]

I like how you can stack them for a compact install. What are your thoughts on running a single battery? I don't run the system much without the boat running.

[David Analog]

The JL Audio MHD and HD amplifiers also have stacking kits. Here's what I've seen with both brands. Stacking is fine within an air conditioned vehicle cabin, cargo area, or even a trunk with a soft bulkhead. But stacking does inhibit cooling. Not good here in Texas during July and August in a sealed port locker. Your summers may be very different.

As you have already concluded, a second battery will offer no benefit while underway. Keep this in mind. Whether or not the boat will still start should not be the determining factor. You can start a boat with less voltage than is a healthy level of depletion for the battery. If you are going to deplete the battery at all, beyond starting, then I would use a deep cycle battery, and perhaps a larger size of battery.