Dave,
I have my Sony (round) speakers mounted flush up under the gunnel facing to the rear. I originally thought they would sound muffled or something but they actually sound quite good. I have another pair inside the cabin also mounted down low and the whole package sounds awesome. I have a Sony Marine deck as well. I orignally was going to mount box type speakers but have a friend with a 21 parker and they always work themselves loose so I went with the flush mount. Let me know if you want to see pics. Randy.