Ahoy chaps, hope you are having a great day.

I want to make a BV 238 ish floatplane with a similar hull structure as the new Sea Plane in the game. It may or may not be known that that thing stays afloat with small floating blocks in the hull.

My build however somehow stays afloat without any buoyant parts (double and triple checked the settings of the fuselage parts). But I want it to have a certain watermark, yet it still stays too high in the water.

Are there some parts that float by default? Or am I doing something inherently wrong?