If a mob destroys the boat the items are dropped on the ground.
The items on the ground will despawn after time unless it is in a workshop aura.
Mobs can also destroy the workbench first and then the boat.

So the only safe place for a boat is in your inventory, unless you build a base.
Which is why this guy suggested this:

Originally posted by Ryzilynt:

Safest place to park a boat is always inside a chest

Only thing is, mobs can attack the chest and destroy the chest which leads back to the original issue.

Keep in mind destroying your boat with a weapon will give you back all the materials to make it again.