I had the same problem on my 150. The problem is they load the hull with a styrophone type. I used a long phillips screwdriver and pushed through the clogged styrophone until water started draining. It's been fine ever since.
If the seat is anything like the newer model than they should be fastened with plastic anchors. The pull straight up and in most cases can be reused. An easy way to tell is to look under the seat through the engine bay (should be right above the gas tank).