I often times hit the water with 4 people aboard. I toyed with building Haka bench seats but first decided to try my luck with clip on sit on top kayak seats. I purchased a cheap Ocean Comfort Plus seat for around $40 and here's the result:
Works great! We gave it a stress test today out on the water with a 160 lb. passenger and the seat gave plenty of comfort. It also helps keep your butt dry as water doesn't penetrate the seat as it does the trampolines (still a wet ride). For a dryer ride on the trampolines we laid down a yoga mat. I was pleased enough to immediately order another seat for the other side. Next I might try building Haka benches and mounting the seats to them.