Best Glue for Planking Float Hulls
 

Try a polyurethane glue such as "Gorrilla Glue" or the polyurethane version "Pro-Bond" by Elmer's. (there's 3 differrent Pro-Bond glues... read the label!)

You'll have to firmly wrap the planking after putting it on... the polyurethane glues like to expand. They are waterproof as far as the bond... not guaranteed to prevent water penetration becuase they foam a bit. The less the added mosture, the less the foam action. (but some moisture is necessary if humidity is below 50%, or it won't cure for a LONG time.)
Side bennefit. the foaming action makes all the sheeting glue joints for you. ;) (more of a bonus when sheeting a wing than with floats....)