I'd point out that marines did use muskets and accompanying bayonets and the boarding pike was used regular sailors.

On the point of armour by the time of the Napoleonic wars the only armour commonly in use was worn by heavy cavalry in the form of a Cuirass, a metal breast plate, and a helmet. Even this armour couldn't stop a musket ball at close range or for that matter grape shot and cannonballs. Heavy armour would have made it difficult to go about a sailor's primary duties in battle or otherwise in the form of working the sails and cannons both of which require a certain level of mobility, dexterity and stamina that wearing armour might reduce. It also wouldn't be helpful seeing if the ships weren't the same height a boarding party might potentially be needing to climb up or down in order to board and enemy ship, even if they're the same height two ships lashed together in the open sea might well be smashing and grinding against each other with the movement of the waves and a sailor would need to be able to easily cross between ships. Additionally at sea keeping armour requires more maintenance with regular oiling and cleaning to avoid rust. Larger shield would be awkward and give little protection from fire arms. Smaller bucklers and targes were occasionally used in the earlier period but these fell out of fashion for the most part by the 1700s in favour of using a dirk or pistol in the off hand.

By extension if no one wears armour much armour you don't need anti-armour weapons such as war hammers and without the protection afforded by armour its not easy to safely use greatswords and longswords with two hands.