Showing up nude in public where there also happens to be warm water, a good wind, 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 do not happen at nudist sites, particularly among adolescents, because that statement is way off the mark.
You commented that you did not mention anything to your wife about your son’s erection in the water (“I never told my wife.”)
The boy is quite 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 is a completely regular boy and there isn’t any need to be embarassed. But do underscore the necessity to behave appropriately if (when) it occurs around others, by not waving it around like a big flag and to only dismiss it if he can not easily conceal it. Not bringing others’ attention to it’s the best course of action. If someone does notice, he should remain cool as if nothing is going on.
And as one poster already implied, ensure that your daughter will not make fun of him if (when) she sees him with an erection. Thirteen year old boys may be http://nudist-video.net/naked-body-it-just-got-hotter-at-the-nude-beach.html about those opinions.
I also suggest having a discussion with “the wife” and acknowledge the information you gave her is not completely correct. She is going to almost certainly spot your son with an erection one day at the resort. She needs to understand he’s having them, and providing correct info regarding the matter will lessen her shock and embarassment. In case your wife understands how likely and normal erections are for a young adolescent boy, she can take it in stride when it does happen and not make your son feel bad about an uncontrollable body function.