Ive worn thongs since about 16 weeks! Im a teacher and on my feet all day - the only thing that would get them down was if they were raised above chest level and sometimes I would use a wet towel if they were sore too. Jus raising them while sitting would keep them steady but not reduce! If that makes sense! My BP has been fine too - if you notice a sudden increase in the swelling or swelling in your face that comes on rapidly - make sure you get it checked but otherwise i hope you find something that helps!
Oh you mentioned aqua aerobics classes too - being immersed in water from chest down helps as well!
FIrst thing Im buying once this bub is out is the highest most gorgeous heels you have ever seen!!!! I literally havent worn antyhing but Havainas for 5 months!