I replaced my stock speakers with MM651UM and installed two extra by the swim deck ( did not caulk, used a waterproof foam tape behind speaker lip, I liked the access to area). I paid $135 a pair for the mm651um. it is only twice the price of the db651. a swim deck install where the speakers get wet was what let me to the MM651UM.
I installed two simple DPDT switches above the throttles and labeled them.
(8489 BLUE SEA WEATHERDECK TOGGLE SWITCH BOOT and 8515 BLUE SEA WEATHERDECK TOGGLE SWITCH - DPDT, ON-OFF-ON)
Wired them to select front port speaker or swim deck port speaker. The other controls rear port speaker or swim deck starboard speaker. No having to worry about ohm's. Works great. When driving we use all 4 speakers inside boat. When swimming we use 2 speakers in boat (starboard side) and 2 speakers on swim deck. Good sound quality and loudness. Total project about $400.00.
Rewire note: Swim deck did not have left/right stereo balance. Rewired so front left is swim left and rear right is swim right for surround sound balance
Installing the port speaker be careful, I am 1/2 inch away from exhaust pipe hose under swim deck (measured 4 times).
All the information I found said the speakers are marine nature in design and can get wet and submerged without problems.
Currently it works and sounds better than I expected, we are not a crazy loud music family, would rather hangout and talk.
My wife is thrilled about sound quality improvement.
I know an amp will sound amazing one day.