Considering that the costume presented is probably for Muslims or orthodox Jews--I have to comment--at least they take the time to make an outfit specifically for an activity that still fits in with their rules of modesty.
I wouldn't be offended or upset if I saw a group of Muslims at the beach in this kind of swimwear. Why would it be offensive or stupid? As long as the material is made out of the lightweight swim material, there's nothing wrong with it from a practical standpoint. I don't think the aim is to be a "witness." I think the aim is to participate in a fun activity and still be modest.
I find it MUCH more offensive to see someone jump in the lake in their regular clothes. (e.g., jeans and a t-shirt)
There's nothing wrong with modesty at the beach or pool. Jeffrey is taking swim lessons right now, and he wears board shorts and a rashguard. All of the instructors, including the guys, wear t-shirts over or with their swimsuits (bright yellow that say "instructor" on the back).
My Baptist cousins wear shorts and t-shirts--I haven't seen a single picture of them at the beach in swimsuits.
Is that actually something we need to ridicule?