When it comes to kayaks, as with most boats, everything is a trade-off. The wider it is, the more stable it will be, and the easier it will be to shoot from, and the less likely you'll be to risk dunking your equipment. I've got a sit-on-top ocean kayak scrambler that is so stable I have no problem paddling with my camera around my neck. I just toss a towel over my shoulder and drop it over the equipment when I paddle to keep the water from dripping on it. I have a larger waterproof bag that I place my camera backpack in when I take trips. The problem with wide kayaks is that they are slow, so if you have to cover alot of area where you shoot, you'll want to buy a faster, more narrow kayak (depending on your paddling experience) and maybe go with one of the covers like Josh mentions.
