All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is mandatory on men and women to cover their ‘Awrah (parts of the body that must be covered in Islam). So, the hidden private parts are parts of the ‘Awrah and must then be covered. As for the thigh for the man, it should also be covered, as it is also part of the man's ‘Awrah.
So, the Islamic swimming suit for the man must be big enough to cover what is between the navel and knee. As for the woman, it is unlawful for her to swim in front of men as to avoid uncovering some of her ‘Awrah. But if the woman is swimming with only ladies, she must cover only from navel to knee.
On the other hand, know that most of the existing swimming suits are not long enough to cover the thigh and might even sometimes cause the uncovering of one of the private parts in front of other people.
Allah knows best.