Everyone aboard, not just the renter
A boat rental waiver signed only by the person paying leaves the majority of the people on the water uncovered. Passengers, friends, and children all need to be on the document, which means the form has to accept a whole party under one booking without becoming a chore.
Capture each passenger's name, and for minors, a guardian's signature. If the boat leaves with someone unlisted, your record does not match the incident.