Showing up nude in public where there also happens to be warm water, a nice wind, 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 do not happen at nudist venues, especially among teenagers, because that statement is way off the mark.
You commented that you did not mention anything to your wife about http://videonudism.com/voyeur/jung-frei-nudist.php in the water (“I never told my wife.”) The lad is very likely able to sense your need for secrecy in http://voy-zone.com/nudist-clip-sample.html and that fact alone will make him uneasy about it, or even embarrassed.
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 totally normal boy and there isn’t any demand to be embarassed. But do accentuate the necessity to act appropriately if (when) it occurs around others, by not waving it about like a big flag and to only dismiss it if he can’t readily conceal it. Not bringing others’ focus to it is the very best plan of action. If someone does notice, he should remain cool as if nothing is going on.
And as one poster already implied, make sure that your daughter does not make fun of him if (when) she sees him with an erection. Thirteen year old boys can be quite sensitive about those remarks.
In addition , I propose having a talk with “the wife” and declare the information you gave her isn’t wholly correct. 
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ViewShe is going to almost absolutely see your son with an erection one day at the resort. She has to understand he’s having them, and providing right info regarding the issue will reduce her shock and embarassment. If your wife knows how likely and normal erections are for a young teen 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.