Duecer, teenage boys get spontaneous erections very often and anywhere they happen to be. Showing up nude in public where there also happens to be warm water, a nice breeze, and naked females romping about is going to make your son’s arousal significantly more likely. I recommend that you refrain from insisting to your wife that erections don’t occur at nudist venues, particularly among teenagers, because that statement is way off the mark.
You commented that you didn’t mention anything to your wife about your son’s erection in the water (“I never told my wife.”) The boy is very likely able to sense your need for secrecy in this matter and that fact alone will make him uneasy about it, or even ashamed.
The next time he gets an erection in your presence, be casual about it. Tell him it’s part of life, there’s nothing wrong with having one, he’s a totally normal boy and there is no need to be embarassed. But do emphasize the need to behave appropriately if (when) it happens around others, by not waving it around like a big flag and to just ignore it if he can’t readily hide it. Not bringing others’ attention to it is the best course of action. If someone does notice, he should stay cool as if nothing is going on.
And as one poster already suggested, make sure your daughter does not make fun of him if (when) she sees him with an erection. Thirteen year old boys can be very sensitive about those comments.
I also advise having a talk with “the wife” and admit the information you gave her is not entirely correct. She will almost surely spot your son with an erection one day at the resort. She needs to know he’s having them, and providing correct information on the topic will lessen her shock and embarassment. If your wife knows how likely and normal erections are for a young teenage boy, she can take it in stride when it does occur and not make your son feel bad about an uncontrollable body function.