Towed sports produce serious injuries
A person on a rope at speed is exposed in ways a passenger is not, and the injuries when it goes wrong are severe. The claim is made against the operator and the owner of the vessel. Underlying watercraft limits are frequently modest relative to what that costs.
Some policies address towed sports specifically
Watercraft policies vary: some include liability for towed activities, some exclude them, and some require a dedicated observer or impose conditions. It is a clause to read rather than assume, and it is the difference between cover and none.
The umbrella follows whatever is underneath
If the watercraft policy excludes towed sports, an umbrella above it generally does too, because it sits above cover rather than replacing it. Both layers have to contemplate the activity before anybody gets on the rope.