While propane has plenty of qualities charcoal is the fuel that has long been used for being able to fire up that barbeque. Some people love the ritual of being able to build up that fire over time with a boat charcoal grill before you have the perfect amount of heat available. For some people, this can be part of the charm of having a grill in the first place.
Propane though by all other measures is better except for the price. With a charcoal grill aspects like being able to control it in windy conditions or having temperature control can be a lot more difficult. Once you have finished charcoal grills are a lot more difficult to clean and you’d also have to throw away the charcoals if they had burnt out.
There is also a safety issue. Charcoal grills are harder to manage and are more likely to get out of control. This is rarely a problem if you’re on solid ground but on a boat, it obviously becomes a lot more of a problem. Charcoal though will cost slightly less over time but gas boat grill beats it in all other aspects.