Check out
www.abcunderwear.com they have all kinds of thongs with all kinds of coverage, designs and materials. Also
www.bodyaware.com some great looks too and they really focus on the soft fabrics. Remember that there is really not any difference between underwear and swimwear when it comes to being that naked in a thong. Only concern is transparency when wet.
I remember being in your situation years ago and was merrily floating on a raft catching rays with a g-string (basically a pouch and non-detectable string holding it on) Music playing and me in a fog. Her dad forgot papers and came home to get them -- I opened my eyes to see him standing by the pool staring at my minisquele garment. No way to cover up. He said if I wanted to wear something like that when alone it was fine, but not when any of his family was home and keep shorts handy at all times in case someone comes. He then hooked a bell to the door to give warning.