Permissive operator cover is not automatic
Auto policies generally cover a permitted driver. Watercraft policies vary far more: some cover any competent operator you permit, some name operators, and some impose age or experience conditions. A guest at the controls of an uninsured operation is the owner’s problem.
The umbrella inherits the underlying position
If the watercraft policy does not cover that operator, the umbrella above it generally does not either, because an umbrella sits above cover rather than replacing it. Both layers have to agree before you let somebody take the boat out.
Age and experience conditions catch people out
Younger operators in particular can fall outside a policy’s conditions even with the owner’s permission. It is worth reading the operator clause once, because it is the term most likely to be breached casually on a summer afternoon.