Muffled sound coming from stereo. Need help.

I have a stereo that includes a 5-channel amp running one 12" sub and four 6.5" speakers. Everything was working fine a couple weeks ago when I tested it. But, now after I took the boat out once, the sound is aweful. It's really muffled and it cuts in and out. It almost sounds like it's not getting very much power too. Does anyone know where I should start looking for problems at? I have checked the power and ground wires already. The power light on the amp comes on like it should. I don't know where to go from here.