Small Passenger Vessel Liability Fairness Act of 2021 This bill revises maritime law regarding the liability of owners of certain small passenger vessels for violations of safety rules resulting in accidents on such vessels. Under current law, owners may avoid liability in some cases depending on the value of their vessels after accidents. The bill requires the Coast Guard to promulgate rules that require owners or operators of small passenger vessels to provide just compensation in any claim for which they are found liable. The bill makes the revised liability provisions retroactive to September 2, 2019, the date of what is known as the Conception boat fire in which 34 lives were lost.