I think having a method to enter the "battle interface" in order to practice ship handling and exercise the great guns would be a useful feature for folks that are transitioning to manual sails or simply getting the feel for a new ship. Learning the ropes in square rigged vessels is arguably one of the most demanding lessons in the game and having to do so against active hostiles can be a bit unwieldy at times. Being able to simply practice the manual sails with some raft targets available to engage would soften that learning curve. Depending on how crew and XP mechanics eventually shake out a small crew XP gain for gunnery practice might be a nice touch as well from a historical fluff perspective.
Restricting it to only the protected waters of the faction capital maybe a method of keeping this from being exploited, either avoid action or setup ambushes in the open world.
Cheers,
JB