The boat must be scheduled and insured beneath it
Umbrellas list the exposures they sit above. A boat that is not named, and does not carry its own qualifying liability policy, is generally outside the umbrella. Households buy an umbrella for peace of mind and leave the single most dangerous thing they own outside it.
Passenger injuries are the serious claim
Most boating liability involves people aboard rather than another vessel. Injuries on the water are frequently severe and the claim is made against the operator and owner. This is the scenario where underlying limits run out quickly.
Towing it is a separate question again
The trailer and the tow are an auto matter, the boat on the water is a watercraft matter, and the two policies have different limits. A household can be well covered on the highway and thin on the lake.