What happens when a big boat collapses?

Boats, ships and vessels do not collapse, they capsize. If they sink to depth with a enough air retained inside they may implode, but this more common in movies than in real life.

When a vessel capsizes, one of three things happens. It simply rolls over and the hulls sticks out of the water. The vessel partially submerges and the remaining air partially floats the vessel or it sinks to the bottom, depending on the depth of water.