Before painting underwater metal, check for incompatibilities. For example, it is usually not recommended to paint copper based anti fouling directly on metal (like rudders). There are metal specific paints (like Trilux) or you can undercoat with a barrier such as Interprotect then use the regular anti fouling. You are trying to avoid any "reaction" between the metal of the rudder and the copper in the anti fouling. So, it also depends on what anti fouling you would be using. Growth around my area is much less than in say Florida. Many around here use a zinc based paint on the prop and running gear. The downside, it only lasts (less than) a year, but it does slow down growth while it lasts. Some use an expensive product called "prop speed" but that also requires a very specific application method. Some users rave about this product, some say not worth it.
Other things to check are: play in your prop shaft (condition of cutlass bearing), visual of prop looking for any damage or dezincification, checking,cleaning, and lubing through hulls, servicing bow thruster (stern if have), clean, polish, and wax hull, and of course, repainting the anti fouling as needed.
Depending on your boat and it's systems , there may be more to do and check, but that is a start and applies to a lot of boats.
