Marc, I'm saying this as kindly as possibly if you have to ask please, don't even try. It may be a 410A unit. although 134A is most likely.
Marine AC units, are like home refrigeration and window ACs, they are factory built under controlled conditions and very unlikely to ever need "a shot"
As they age, electrolysis, rough, handling,cleaning with sharp objects may cause it to loose refrigerant but it's more likely to not work at all or freeze up. Low water flow, blocked air filter, clogged heat EX, water not pumping, closed or forgot to open the water valve/s all can cause problems making the lay person think it needs a shot of gas
Please try to describe what is happening, not happening, age, model number, pump, strainer, locations, . Heat exchangers on AC units need cleaning. White vinegar is one of the best "acids" to use.
Hope this helps!