Saw a fair bit of naval usage. Fireships (literally ships that had been set on fire and generally sailed towards a fleet at anchor) were pretty common from the 16th century onwards.

Your other popular option for trying to set ships on fire was heated shot. That's your standard round shot heated. Again 16th century onwards. A rather less popular option was Saw a fair bit of naval usage. Fireships (literally ships that had been set on fire and generally sailed towards a fleet at anchor) were pretty common from the 16th century onwards. A less popular option introduced by the royal navy in the 19th century was Martin's shell. Molten iron was poured into the shell and then fired at the enemy. Well in theory anyway. In practice I'm not convinced it was ever used in anger.