LOL, I was literally about to post the same question..
My 1998 1800SR has two speakers, one under the console and one the other side. Interestingly the boat has wiring for two speakers at the back (running along the sides) already in place just not connected to anything. Additionally the previous owner nicked the radio so all I have is a sony wiring harness
Was considering getting a bracketed rectangular type speaker for each side at the back and point them downwards?