It has come to my attention that some people appear confused about this, and as the weather makes many of us wish that laying poolside was all we had to accomplish today, I expect this to continue.
So, I think mostly, people understand that bathing suits are generally worn at the pool or the beach. (If you did not already know this, um, now you do.) Sure, there are rivers and rock quarries other places where people swim, so perhaps we should say bathing suits are for swimming, but, of course, no one is going to make you swim.
Now, if your plans for the day look something like this - swim (or lounge water adjacent), eat, repeat, I think we all understand that getting into and out of bathing suits can be a challenge, so on or near a lot of places where swimming occurs, there are casual eating establishments that have a lax approach to the general expectation held my most eating establishments that their patrons be clothed. There are also bathing suit cover ups that are generally sheer and/or lacy, but allow the impression (at least at first glance) that you are dressed.
Some places, particularly beachfront will extend this radius to anything that is say, along the boardwalk.
However, once you get beyond this radius, the normal rules resume. Now people expect you to be clothed. And certainly no one should be checking to see if you have underwear or a bathing suit on but this also means, I, person unknown to you, do not need to know whether you have underwear or bathing suit on. Here's how I will become aware (whether I want to or not.)
-If you have a bathing suit on in place of a shirt. No, your bathing suit does not look like outerwear. It might look like a bra, but given that a bra is also not appropriate out and about wear, the bathing suit-ness of it all is not an improvement.
-If you have on a bathing suit cover up. Cover ups sometimes look from afar like other items of clothing, but as previously mentioned they are lacy and or sheer. This means that I (and everyone else) can clearly see what is under them, again, whether I want to or not.
Now, it may seem like it's confusing to figure out where this swimming adjacent radius ends. My suggestion is two fold.
One. When in doubt, wear real clothes. Street clothes.
Two. If you cannot see the thing in which you might swim, you are probably pas the radius, so put on the real clothes.