I added a radio to my boat of simular design. I cut the radio into the front of the rear seat as well as the speakers into the same area. I used a weather proof radio cover that I picked up from a local marine dealer. Wasn't to expensive. The only problem is the rear seat area was filled with foam. Had to be careful to cut out enought to allow for cooling as a lot of the new units do give off a bit of heat. The foam made a good enclosure for the speakers and it sounded good. If you go with a cd type there not made for the rough bouncing that the water will give it. The Card type is a good choice.
John