As a religious person (different religion though) I have some sympathy for your view, but I do think that those people going to baptisms should be at least a little aware of how they look to outsiders and perhaps dress with modesty in mind.
I'm not saying that they need to plan for an inspection from a wetlook fetishists forum, obviously, but take a look at that first girl. I mean, if you know you are going to get dunked as part of a religious ceremony, why would you wear a tight white t-shirt and a bright red bra, unless you were a bit of an exhibitionist to start with?! She might as well be saying "look at my boobs!"
As an aside, I'm curious as to why baptists all get dunked in regular clothes, rather than swimwear, anyway? Taking the wetlook aspect out of it for a moment, it does seem to me that it would be appropriate for people to wear swimwear given that they know they are going to get wet. Alternatively, if they idea of avoiding swimwear is to avoid showing flesh for modesty reasons, I would have thought that there would be a standard type of dress to wear, but what we see in this forum is folks being dunked in all manner of clothes. Anyone know the answers to these questions? I am genuinely interested!