Petrol is extremely explosive. Most small boats are hand fuelled with petrol.
An LPG cylinder, used to heat a Stirling, does need standard Boat Safety certification.
Sealed, refillable LPG cylinders are far better than hand poured petrol tanks.
The crucial certification of a steam boiler is not involved.
The Stirling engine can meet the needs for a small boat and also deliver a "better" boating experience.