A quick assessment can be accomplished by checking temperature difference or delta t or "split" across the indoor coil. You should see a cooling delta t of about 20F, heating about the same or higher, depending on variable conditions. A higher split can be an indicator of poor air flow through the coil, lower split may accompany dirty condenser section (the seawater coil), inadequate charge, too much air flow (not ususally seen). A non-invasive 1st line service is to make sure the exhangers are clean, filters clean, and adequate seawater flow, no obstructed strainers. If you can locate the suction line, the larger line returning to the compressor, or if a remote fan-coil, the larger, insulated line in the refrigerant lineset. That line in cooling should be chilled, sweating where exposed, cold to the touch. If it's not, there may be a refrigerant issue, and that diagnosis should come from a knowledgeable service tech. Refrigerant systems do not require periodic "topping off". They are a hermetic system, meaning they are sealed. If refrigerant is low, there's a leak. It will not heal on its own, a repair is the only fix. As David mentioned, a reverse cycle system can suffer from a stuck reversing valve, that DX is in the realm of a service tech. Cleaning of the condenser/seawater coil can be a DIY project, how difficult is contingent on ease of access. Basically, a recirc of cleaning agent such as salt away, muriatic acid, or barnacle buster type agent will make a significant improvement, and doesn't require opening the refrigerant system.
