In my experience if there's a loop-de-loop threshold, that indicates a deeper problem that will show up later.
Just imagine your boat is a model being held up by a string from its center. If you put your finger on the propeller and pushed the model forward, which way would it turn? For example if your propellors were on the left side of your ship pushing it would cause the model's nose to tip to the right. Since your propellors are on the bottom, imagine if you pushed the model forward from below. It would cause the nose to tip upwards. This causes the props to change orientation upwards and thus the boat goes out of the water.
A good boat should have the center of mass *slightly* lower than the propellers. That way the boat is constantly tipping down. But as the front of the boat submerges, the buoyancy increases, and as a result the upward force on the nose increases. In calm water at any (reasonable) speed, the nose will always remain stable. Boat speeds up, buoyancy increases. It stays in equilibrium. Your boat doesn't stay in equilibrium, because as the nose exits the water the buoyancy decreases but so does the water resistance. Water places greatly more drag on a vehicle than air. As your vehicle noses up, it wants to do two things. The sudden decrease in buoyancy makes your boat want to fall. But the sudden decrease in drag makes it want to speed up. At low velocities falling wins and your nose goes back down. At high velocities going forwards wins, and since your nose is aiming upwards that means the boat goes upwards. That's your loop de loop threshold. Put a bunch of lead at the lowest and most forward point on your vehicle, I guarantee it will speed up despite being heavier.