You probably unscrewed the bolt too much so that the nut fell off. If you unscrew just enough to loosen, but not drop off the nut, you can move the seat and tighten back up easily. If the nut comes off, you need to find the small slot in the seat that you can get to the nut through. Peel back the hip pads... there is usually a hole in the seat to get to the nut. Be prepared for some difficultly... the small slots make it so the procedure would be simple for elves with pencil thin fingers, but cumbersome for humans. To find the nut, you may need to tilt and shake the boat to get it to come out.
Best luck I have had is to hold the nut in one hand and a screwdriver in the other. With two fingers, try and get the nut on the head of the screw by feel (hard to see inside) and screw on the bolt to get the bolt to mate up with the other hand.
Note: I have not seen the attachment of this specific boat, but I've done this on many other boats and they have all been similar.