ELI5 i'm wandering why ships have such a small part of them underwater

I'm just wandering why do shape have so much hight above the sea line, wouldnt they be more stable if most of the ship was underwater?

I mean i understand for bigger ships this wouldn't be a good ideea because of the risk of hitting the bottom of the sea.

But for a smaller vessel if it had a water tight deck that is 1m above the water, and the rest of the ship would be under it, wouldn't it be incredibly stable in rough waters?