What is the ruling on male and female mahrams swimming together in swimwear in a private pool where no one can see them?

It is permissible for women and their mahrams to swim if Islamic regulations are observed, including covering the awrah. However, a tight diving suit is not considered satir (covering), as it may cling to the body when wet and become a source of fitnah (temptation). It is obligatory to block all means that lead to the unlawful (sadd al-dharai'). If your ignorance of the Islamic ruling prevented you from denouncing them, then there is no sin upon you. However, if shyness was the impediment, then it is a blameworthy shyness that prevents one from fulfilling an Islamic duty. It is possible to allocate a time for women to swim alone and a time for men to swim alone, and this is more cautious.