A rough "rule of thumb" for sizing marine
air-conditioning (for use in sub-tropics) is between 14 BTU/H (below decks) to 17 BTU/H (above decks, glazed) for each cubic foot of interior
conditioned volume (living space).
A professional will size the air conditioner based on the latent cooling
load (which considers the relative humidity of the air), as well as the sensible cooling
load (which considers the outdoor & indoor design air temperatures) for your boat & location.
A (theoretically) perfectly-sized air conditioner will run continuously during the hottest 2.5% summer design (outdoor) temperatures.