Easy enough to store a couple of tanks and gear, as well as finding compressors for fills. In my mind it would depend on the expectations of the one diver on board. If he simply wants to dive off the boat at locations where you happen to be anyway, I don't see any inconvenience. If he expects you to take him to a dive location while everyone else on board will have to sit and wait, that would not be acceptable. Nothing wrong with having a diver on board to retrieve items dropped in the water, as well as cutting line off the propeller if necessary. Solo diving would be a personal choice.