I've tried a couple ways, none I really liked.
I would say for a aircraft that is already built, minwax... I've had limited success with it (it does seem to yellow a bit though.)
I have a couple blank sheets I was going to try and see if the cut with the laser okay but then I remembered hearing maybe hotglue does not sticks to the minwaxed sheets... So I might try using the foaming gorilla glue for those builds... (hey, maybe I'll use those for the floats!!)
I tried using colored packing tape on my seaduck build... it's neither pretty, or waterproof... But I have thought about recycling it for the motors, so I might give her a baptism and fly her till she falls apart.
Flite Test WP foamboard is all around the best option for seaplanes I think (been my experience anyway...)
Yes, the Scout is still alive (I think I've got that Gryo figured out now, so you'll see them on other builds (one for my son first.))
Now that you've got me thinking about floats I might update her and take her the same day I drown the sea duck.
I need to waterproof the esc and such first though!