Where would they spawn? When I explore the map (I mostly play gigantic Americas map) the map is filled with native tribes and their villages. "Barbarians"/Wild Animals are hostile to the natives, too - so would those native raiders start raiding the next natives? How would they know which city is lightly defended? If they approach to scout and attack only lightly defended ones, sure - but please no beelining to the one city without a garrison. Can player ships do that too? If not, no. Not "too far" - simply when outside your own cultural borders. Only if the AI understands that and does not send out ineffective units for not understanding that it would need to supply them. Historically usually every ship had a carpenter and a small store of wood on board for repairs. Masts broke, cannonball splintered through the planks etc. Small repairs - or a limited amount of repairs should still be possible outside ports.