I’ve worn thongs to swim laps and they’ve come over to me while swimming and asked me to not do it again. Every time I’ve done it at that pool. The silly thing is that it was always like 6am when no kids are there. This is in LA.
Nowadays, almost all girls over 15, who train seriously in swimming, wear very cheeky swimwear which become a thong when they swim.
The major brand sell for them very cheeky one piece swimwear or cheeky bikini and sometimes thong.
It is impossible for lifeguards to ask a man to not wear a thong without doing gender discrimination.
Otherwise, the best answer is to swim, to swim and to swim.
If you do a serious swim session, lifeguards and other swimmers will consider you as a "serious swimmer", no matter if you wear a thong or a brief.
If you are a poor swimmer, you will be considered as a "thonger", a "sun bather" but not a "swimmer"...
This is unfair but it is valid for men and women. In my country, the consideration given to you will depend on your swimming level.
That is the reason why, when I arrive in a new swimming pool, I hurry to start swimming. That way, others consider me like a serious swimmer, no matter what swimwear I wear, no matter my swimwear is a thong.