What is the ruling on men entering a swimming pool while the area from the navel to just above the pubic area is exposed?

It is not permissible to expose the area between the navel and the knee, as it is awrah (private parts) that must be covered from anyone for whom it is not lawful to look at it, whether in a swimming pool or elsewhere. As for the navel and the knee, they are not part of the awrah.