Living in proximity to beach does not mean the other person is willing to meet someone for the first time there. Did you somehow miss the part about how they're dressed when meeting someone to talk about something important?
Do you think people in Finland go and meet in sauna someone they've never seen before and talk while they're butt naked in there? No, no they fucking don't.
No one would challenge me on this irl, but no, I just have to accommodate someone online and explain things to them like they're autistic. You ARE socially deranged.
If you live in Florida or California near the beaches, or in places like St. Torpez or Nice in France, then you would 100% have no problem going to the beach with someone the first time you meet them and even to talk about important things. They have weddings and even funerals at the beach, still wearing beach clothes and even some chicks in bikinis. It's not that big of a deal or complicated. Yeah, they would 100% be willing to go and talk about something like a dead parent at the beach. And as for Beth, she is trying to blend in and go along with the others, so she'd go along with it too because she wants to accommodate them and make sure they like her. And, if I remember the scene correctly, Beth is also apprehensive and scared to tell the Mc the truth, so she also keeps putting it off and actually uses not wanting to ruin their time at the beach as another excuse not to bring it up, so that's another reason she goes along with it.
Anyone who has lived or even visited people near a beach town can confirm that people who live near the beach go to the beach with new people all the time, even for important events it is completely realistic. Also, for people who live in those types of areas, they don't see swimsuits and bikinis as immodest or inappropriate at all. You equating people being naked in a sauna to going to a beach and wearing normal swimwear is weird, but also wrong, people in Finland actually do take new people they meet to saunas, it actually is a pretty normal practice, an not all saunas are nude, some you have to keep covered. It's like a Japanese onsen, some are nude, some you need to wear swimsuits, some are gender divided, some are gender mixed, and they are something that people go to as a normal place to go even for formal business meeting and such.
Whatever hang up you have is the problem my dude, no one else. Even IRL no on else would have a problem with it. No one else here is "socially deranged," or acting autistic. In fact, you're the one who seems to have never been to a beach town in your life or traveled at all or cares to actually understand other cultures and norms. You're missing the normal social behavior of understanding that your little bubble of your mom's basement isn't the whole world. You getting uncomfortable at chicks in bikinis at a swimming pool doesn't mean normal people wouldn't go the beach for everyday activities like getting to know a new person or show a new friend the stuff you do around town.