On the good side, OP, your son exercised some good judgement. Hopefully, this will make it easier on your anxiety, the next time he wants to travel with friends.
It looks like he was more interested in bikini watching versus hardcore partying. Believe me the serious drinkers would NOT be deterred by comparatively cold, damp weather--it would just be an excuse to stay inside to drink, versus outside. So, at least some of your concerns were unfounded. Or at least, less founded, if that's a phrase.
Parenting young adults is tough. You're doing the best you can to give him roots and wings. Keep going!