Having boat insurance doesn't mean it will always cover the cost of your boat sinking in the water especially if you boat sinks due to corrosion or anything else creating holes in the boat.
Now if the boat were in an accident on the water and that caused the boat to sink then if you have full coverage boat insurance then the boat insurance might cover the cost of the boat sinking and any other damages caused to the boat during the boating accident.
Usually though most cases of a boat sinking aren't covered by boat insurance.