I read you post this past week and noticed no one replied to it. I thought about it all week and decided I'd try to address some of your issues regarding takeoffs, water handling, and waterproofing, but will leave the engine questions to someone with more expertise.

Regarding water spray getting into the prop, let me start off with two words:

"Chine rails".

The problem you are experiencing is common with model seaplanes of the "flying boat" or "flying hull" or "planing hull" type. When you accelerate the water is forced out from under the hull and up and back into the prop. What you need to do is redirect that water and you do that with "chine rails". The chine of a hull (whether it's a seaplane or a boat) is the point or area where the hull transitions from the SIDE to the BOTTOM. This is called the "chine line". By adding a rail or strip of material to this line, you can change the direction of the water as it moves out from under the hull.

I have attached a drawing I hope will be easy for you to follow regarding how to do this.

I once highly modified a Jetco Navigator from a single-channel rudder-only model to a 4-channel model. I built it as a true amphibian - with knock-off landing gear for use on the ground. The tail wheel strut incorporated both a wheel and a water rudder for steering. This model worked exceptionally well but suffered from the same problem you describe - the spray was always getting into the prop, slowing the engine down so I couldn't take off. A friend with boat building experience (full size boats) told me about "chine rails". They were not difficult to install, and refinishing to that area of the model was minimal. The chine rails made all the difference in the world! Takeoffs were never a problem after that.

You mentioned not having a "totally calm day" to attempt a takeoff. Believe it or not, most seaplane models don't do well on calm water, unless they are extremely overpowered. Takeoffs are easier on water that's a little rough. Also, use "up elevator" only enough to get the model up on the step, then go to "neutral" and wait until it's moving at a good clip before adding "up" to come off the water.

Regarding your problems with water getting into the hull, would it be possible to seal the hatch and access everything only through the cabin? The more openings you have, the greater the chance of leaks. I even put my aileron pushrod exits on the TOP of the wing instead of the traditional bottom location. Same with the rudder and elevator - TOP pushrod exits (above the stabilizer) instead of underneath. All pushrods were the "tube within a tube" type - no open oval holes in the fuselage with a bent wire pushrod for seaplanes!

Here's how I sealed the opening (cabin) in my model but I believe you could use this for a hatch, too. I could never get silicone to stick to a painted surface - it would always let go sooner or later (usually sooner). This method works for me. Using an awl (or small drill bit), make a lot of holes AT DIFFERENT ANGLES in the top of the cabin (wing seat). Cover the bottom of the wing with Saran Wrap. Make sure it's wrinkle free - smooth it out and tape it down (tape outside the area where the wing will mate with the cabin). Put a good bead of silicone on the top of the cabin and WORK IT INTO THE HOLES you punched with the awl. Immediately put the wing on and fasten it (whether it's rubber banded or bolted). Let the silicone cure completely. When you remove the wing and the Saran Wrap you'll have perfect seal that matches your wing exactly. The holes that you made at different angles, "nail" the silicone gasket to the wing saddle so it doesn't separate later.

I always wrapped my battery pack and receiver in a plastic "baggie" with rubber bands put on tightly around where the wires exit. While I have had excellent results keeping water out of my model using these methods, I want to say that no matter how carefully you waterproof everything, at some time YOU WILL get water into the hull! It's a fact of life. It's the nature of model seaplanes. When it happens, your receiver MAY short out. But, not to worry. I have found that you simply need to take the receiver and battery pack out, drain the water out of the model, and lay the electronics out in the sun to dry for a little while. Once everything is dry, you just put it all back in, the electronics start working again and you're back to flying!

By the way, while on the subject of waterproofing, there is one more area you need to be concerned with and that is windshields. They have a nasty habit of "caving in" under pressure from all that spray that hits them. Most model seaplane kits compensate for this by design or providing heavier windshield material, but if you are scratch building, make sure you use a little heavier acetate and that the construction around the windshield is very strong.