Houseboat Stereo Input, AC vs DC Power, Zoning, Bluetooth, etc

[CoMtnBu]

For the rooftop I'd wager you'd be happier with more pairs of Parts-Express, Kicker, or Yamaha's in the 100/pr range than more expensive ones.  

JBL makes nice outdoor stuff too, but you need to get into the 500/pr for the 8' woofer, the 5.25's are underwhelming for what they cost.

I'd keep the Denon for the 5.1 in the cabin.  Then add two amps.  One to replaced the output on the existing head unit, whatever the selector switch will take power wise.  Then tie in a second amp to feed the speakers on the roof top just remembering whatever you add up top will probably be 8ohm.  Though you could wire them to get down to a 4 or 2ohm load and use a 5 channel amp for everything.

With proper setup, you should be able to control most functions from your phone.  May need to add a remote volume control the amp feeding the roof top.  Some amps have RJ connectors so you could potentially route that up top, or add a separate volume attenuator on the roof.