Peter is right.
House of Balsa makes the chea-pass10 seaplane. It is boxy but reported to fly well. Probably rendered subliminally unattractive by the uncomplimentary name.

Sig/Herr instructions specifically warn against using a motor larger (heavier) than the Norvel .o6. The Aquastar is mostly famous for diving underwater as soon as the throttle is opened. Anything that makes it heavier aggravates the submarining tendency. The .06 is probably as powerful as your .10 anyway. Keep the Aquastar light, and build in the spray rails (chines) to help it get on top of the water.