Showing up nude in public where there also happens to be warm water, a nice 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 places, particularly among adolescents, 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 requirement for secrecy in this matter and that fact alone will make him uneasy about it, or even ashamed.
The following 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 is a totally normal lad and there is no demand to be embarassed. But do highlight the necessity to behave appropriately if (when) it happens around others, by not waving it about like a large flag and to only ignore it if he can not easily conceal it. Not bringing others’ focus to it’s the top plan of action.
If view does notice, he should remain cool as if nothing is going on.
And as one poster already implied, be sure your daughter doesn’t make fun of him if (when) she sees him with an erection. Thirteen year old boys could be very sensitive about those remarks.
In addition , I advise having a talk with “the wife” and acknowledge the information you gave her isn’t completely right. She’ll almost absolutely see your son with an erection one day in the resort. She must understand he is having them, and supplying correct info regarding the issue will reduce her shock and embarassment. If your wife understands how likely and normal erections are for a young teen boy, she is able to take it in stride when it does happen and not make your son feel terrible about an uncontrollable body function.