Your homeowner's policy covers Personal Liability and Medical Payments involving the ownership, maintenance, loading and unloading of a boat. This coverage, however, is limited to: 

Boats with inboard motors equal to or less than 50 horsepower,  Boats with outboard motors equal to or less than 25 horsepower, or Sailing vessels fewer than 26 feet. 

You can remove these limitations by buying endorsement HO215. This "policy-add-on" costs an additional premium and is subject to your company's underwriting rules and guidelines. 

Note: Outboard motors of more than 25 horsepower are covered by your homeowners policy if: (a) you owned them before getting the policy and you declared them when you applied for the policy; or (b) you bought them during the policy period and sent the company written notice to insure the outboard motor within 45 days of purchase. 

Premiums are typically based on the type and length of the boat plus the motor type and horsepower. Companies also may take into account: 

Age, value and condition of the boat pleasure or commercial use boat storage location 
owner's driving record and boating experience prior claims 

Many companies offer insurance discounts for completion of certified safety courses.