It wasn’t a Yes/No question, Vikingbolt.
But I would go with none, personally. I don’t really care to see ponies in swimsuits.
As for what the writers and animators would want, I can assure you that it wouldn’t be anatomically correct. If that was the case, then the doors wouldn’t have doorknobs and cups wouldn’t have small handles. And it would take away from actually paying attention to what’s happening in the episode.
Also, there wouldn’t be much need to wear anything to the beach anyway. Humans wear clothes to cover up. Pony wear clothes at fancy events and the like. They serve no purpose outside of fashion. So if someone were to wear something, then it would likely be Rarity.
There’s a chance that they’d wear human-based outfits. They’ve already referenced nudity explicitly (“Beg pardon, Rarity, but, uh, we don’t normally wear clothes.”) and implied shame when having clothes/covering removed (i.e. When Fluttershy had her cover removed from her by Rainbow Dash in the cider episode that I never bother to memorize the title of.)
But I’d figure that the only pony to wear anything would be Rarity, and it would be something that would be “modest” by human standards (e.g. a one piece swimsuit with a skirt, or a sundress that one of the ponies gets wet, because Rarity would not want to get her hair wet.)
Also, an image to tl;dr it:
I don’t think you’ll see this, but it could happen.
I think you’re more likely to see this.
With something like this somewhere.
But seriously, this is what I expect to see.