Here in MO, your tow is insured for liability only if it is attached to an insured vehicle. If you want replacement or Comp, otherwise known as "full coverage" on the towed item, you need to purchase separate policy. There are some states that are starting to require a liability policy while the boat is floating on the lake I believe. Once again, if you are going to operate a Motorized boat in another state, you need to check the ON THE WATER RULES for operating a boat. That would be available on the States Water Patrol site or Dept. Of Revenue.
And, it has to be licensed, titled, and stickered in at least some state in a current year for operation. Same applies to the motor, trailer, and possibly the trolling motor in some instances.