I bet it has great buoyancy and is difficult to sink. They onyl open it on the ship, after washing off hypergol remains and going over it with a chemical sniffer.
They only land in good weather. I've watched many livestreams of landings, it always looks calm. The people attaching the ropes don't seem bothered. The astronauts inside are not too sea sick.
These capsules don't look that seaworthy, and sinking in one seems like a really terrible and expensive way to go - like that Mercury capsule.