What is the ruling on swimming in a pool frequented by young men whose knees and navels are visible, and is it permissible to go there while lowering the gaze as much as possible?

There is no objection to a man swimming in a pool with other men if they are covering their awrah. The boundaries of a man's awrah, according to the majority of scholars, are between the navel and the knee. The navel and the knee themselves are not part of the awrah.