Your boat weighs too much. Having the entire hull covered in boat boards is not a guarantee that the top of the watertight hull of your boat is above water. If the height of your watertight hull is less than your boat’s draft, then your boat will sink.
How this works in the code:
Your boat displaces its 1 cubic meter of water per metric ton the boat weighs, from basic physics.
The width of your boat is roughly 0.5m * the boat’s width in tiles.
The length of your boat is roughly 1m * the boat’s length in tiles.
Your boat is assumed to have a vaguely rectangular hull form that displaces 7/12 * width * length * depth cubic meters.
Therefore, the draft of your boat is 12 / 7 * weight in tons / width / length.
Your boat has a watertight hull height of 0.3m + 0.5 * tiles with boat hulls / total tiles.
If draft is higher than watertight hull height, your boat sinks.
If your boat is sinking and you’ve put a boat hull on every time of the boat, your boat weighs too much. Either lighten the load or increase the length or width of your boat.
And yes, I am aware that super heavy CDDA deathmobiles made from heavy duty frames with fully loaded cargo carriers on them do not float very well. I’m not going to apologize for that, either.