All boats are fully insured during the rental period. You deposit an amount equivalent to the insurance deductible of the boat directly to the boatowner at pick up. When you return the boat in the same condition as when you got it, the owner will return the entire deposit amount if not something been broken / damaged during the rental period. Remember that you are responsible for the boat during the rental period and that it is maintained/ maneuvered in accordance with applicable maritime rules.
Remember that the boat should always be returned with full tank, tidy and generally in the same condition as when you got her. There may be exceptions, YOUR boatowner tells you about what rules apply when you sign for the boat.