Showing up nude in public where there also happens to be warm water, a good breeze, and naked females romping about will make your son’s arousal significantly more likely.
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 extremely 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 existence, be casual about it. Tell him it’s part of life, there is nothing wrong with having one, he is a totally ordinary lad and there’s no need to be embarassed. However do stress the requirement to behave appropriately if (when) it occurs around others, by not waving it around like a big flag and to just ignore it if he can’t readily hide it. Not bringing others’ focus to it’s the best plan of action. If someone does notice, he should remain cool as if nothing is going on.
And as http://x-pot.com indicated, make sure your daughter does not make fun of him if (when) she sees him with an erection. Thirteen year old boys may be quite sensitive about those remarks.
In addition , I advise having a conversation with “the wife” and acknowledge the information you gave her is not totally correct. She is going to nearly surely see your son with an erection one day in the resort. She needs to understand he is having them, and providing right information regarding the topic will lessen her shock and embarassment. If your wife understands how likely and normal erections are for a young teenage boy, she can take it in stride when it does happen and not make your son feel awful about an uncontrollable body function.