"I find nothing immodest about a pair of jeans and a nice shirt."
I don't find anything immodest about jeans, either. Honestly, I feel better covered in jeans than a jumper. My biggest problem is swimwear. I can't find anything for my 12 year old daughter that doesn't expose a lot of skin. The only other alternative are the swimsuits in some the the homeschool magazines but I think they are to old fashioned looking and would draw more attention to her and embarrass her.
I think for swimwear you have to just accept that it's a very particular situation, one in which wearing, basically, underwear is proper attire and then go with the best of a bad deal. It's always possible to find a one-piece; my swimsuit is actually a two piece top and little shorts. I haven't worn it in a year though.
Jeans are wonderful. I can haul around computers or trace wires under desks - which I am called on to do occasionally in my work - without worrying about someone looking up my skirt.
Have you tried Lands' End for swim suits?