Get some duct tape and wrap it around that tab to hold it, or get someone to depress it for you. Put a section of 2x6 under the seat and hit upward with a hammer!
Gently at first, then if that doesn't work Beat the crap out of it! Don't destroy anything though.
Is it both the front and rear seats or just one thats stuck? If you got one out and just cant get the other this method of "convincing" should work. If you are stuck on both it might be operator error. Do you still have the number of the guy you bought it from? You could call and ask him, or ask a boat dealer that uses the same mounts. They are pretty common. I have seen them on several boats.
Hopefully some one that has them will chime in.