secret squirrel....I am in the same positon as you ,just finishing off my first float plane....jrf makes a comment about your weight I have a piper tomahawk that is 9 lb with a wing loading of 33 oz per sq ft flying with a OS 46 engine.It flys very nicely and handles the wind fine.I used to build light but have since started to put a bit of weight into my models and they handle a lot better.I cant see why a ST 75 wont handle 10 lbs. A mate of mine flys a scale DC 3 with two 25s on and it weighs 10.5lb. he had an engine out and flew two complete circuits on one engine.It just proved to us at the strip that even a small amount of power when handled right will keep an aircraft flying.
water does seem to add a little extra drag...try it and see ......... Mintie.