I have no idea of the size of your boat, but I have used the Mtronics sound board (choices of engine type) together with a pair of Dayton Exciters/transducers ( EX25VT 4 Ohms) which are 2” in diameter.
My boat is a Tug only 21” long with 8” beam. Volume is fine and exciters easy to accommodate. All the best. NPJ