Change the Monitor vessel’s name to ‘Bomb Ketch’ or ‘Bomb Vessel’. Monitor is such an odd name for it as it’s not a ‘Monitor class’ ship (which is incidently closer to the existing Ironclad vessel). Bomb Vessel/Ketch was the historically-accurate name.
Sailors are Shock Infantry (occupying the same role as the Pirate and its new tag, albeit as a non-outlaw, slightly weaker unit). Available to all Euro/NA civs, as well as all other civs when they have access to Royal Embassy (i.e on Euro maps) and trainable via the Docks (so water maps only), the Sailor gives Euros a cheap shock infantry unit armed solely with a cutlass. When garrisoned in warships, they boost HP as well as very slightly up the Rate of Fire. The downside is that whilst your warship(s) may be stronger, you’re using up precious population space.
Tag/Class system for ships
A big one, but ships should have tags in the style of land units, with a better counter system. I’d just like to see naval warfare fleshed-out so that all ship types have a more clearly-defined role.
This should be a part of just about every civ’s naval roster of units. Setting fire to a wooden ship/boat and sending it on to an enemy vessel was a fairly universal tactic. As a proper line of unit, it should be countered by Light Warships (a tag from the suggestion above this one, which covers the light, exploration, first tier military ships). Native American civs would get a lighter version in the form of fire canoes.
British Unique unit - a Rocket-launching ship. Fires a broadside of 32dpr rockets but has a long cool down. Bombardment class ship, so vulnerable to light ships (again, using the tags, so that first tier ships have a bonus against all these utility ships). Does far less damage to buildings however makes and enemy units less resistant to your other units damage (the fear and panic rockets cause).
Turtle - submersible
US unique unit. If Italians can get a slightly fictionally tank, why not allow the US to get an actual, one-off submersible naval unit? Available with a shipment, the Turtle can be used to scout thanks to a Stealth ability (on a timer). It can also attach explosives via a ‘melee’ attack though cannot Stealth during the attack.