Showing up nude in public where there also happens to be http://videonudism.com/Nudists , a nice wind, and naked females romping about is going to make your son’s arousal significantly more likely. I recommend that you simply refrain from insisting to your wife that erections don’t happen at nudist sites, especially among teens, because that statement is way off the mark.
You remarked that you did not mention anything to your wife about your son’s erection in the water (“I never told my wife.”) The lad is very likely able to sense your requirement for secrecy in this matter and that fact alone will make him uneasy about it, or even embarrassed.
The next time he gets an erection in your existence, be casual about it. Tell him it’s part of life, there is nothing wrong with having one, he’s a completely regular lad and there’s no need to be embarassed. But do underscore the need to act appropriately if (when) it happens around others, by not waving it about like a huge flag and to only dismiss it if he can’t easily conceal it. Not bringing http://nudistsass.com to it is the top plan of action. If someone does notice, he should stay cool as if nothing is going on.
And as one poster already indicated, make sure your daughter will not make fun of him if (when) she sees him with an erection. Thirteen year old boys can be very sensitive about those remarks.
I also propose having a conversation with “the wife” and declare the information you gave her is not completely right. She’s going to nearly certainly spot your son with an erection one day at the resort. She has to understand he is having them, and providing correct info regarding the subject will reduce her shock and embarassment. If your wife knows how likely and ordinary 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 terrible about an uncontrollable body function.