I use a marine grade velcro by 3M to attach my NK Speedcoach, never a water problem with the adhesive it uses. It is available from many marine supply stores but I suspect that if you used this stuff to attach a seat, you would never get the seat out again...

However, the link below leads to all kinds of velcro goodies....

: Greetings,

: to finish outfitting our Artic Terns, my wife and I installed a small
: Thermarest cushion as a seat (comes with the Pygmy kit). We used velcro
: glued to the kayak botton and the seat bottom (we used the glue provided
: in the velcro roll).
: The Velcro is very strong, but the problems that we have is the glue that
: comes with the velcro, it becomes weak very fast with water. End result:
: at the end of a pddling day, our seats almost float (specially after
: practicing rolls).

: Question: does anybody have any recomendation on velcro brands to use, that
: won't detach very easily with water ? The one we're currently using was
: bought from Home Depot (industrial strength, yeah), sorry, cannot remember
: the brand. I'd prefer not to use some glue like contact cement that I
: cannot later remove, since I'm eyeing Joe's seats for the future, but I'm
: going to the Apostle Islands in 3 days and need to have some solution
: that'll last for a long weekend.

: Thanks in advance for your help,

: Hugo