Showing up nude in public where there also happens to be warm water, a nice breeze, and naked females romping around 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 happen 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.”) http://partnerpost.net is quite likely able to feel your need for secrecy in this issue and that fact alone will make him uneasy about it, or even embarrassed.
The following 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 ordinary boy and there’s no need to be embarassed. However do underscore the need to behave appropriately if (when) it happens around http://wildnudists.com , by not waving it around like a big flag and to just ignore it if he can not easily conceal it. Not bringing others’ focus to it is the best course of action. If a person does notice, he should stay cool as if nothing is going on.
And as one poster already implied, make sure your daughter will not make fun of him if (when) she sees him with an erection. Thirteen year old boys may be quite sensitive about those remarks.
I also suggest having a conversation with “the wife” and disclose the information you gave her isn’t totally right. She is going to almost certainly see your son with an erection one day at the resort. She needs to know he’s having them, and supplying correct info regarding the issue will decrease her shock and embarassment. In case your wife understands how likely and normal erections are for a young teenage boy, she is able to take it in stride when it does occur and not make your son feel awful about an uncontrollable body function.