I doubt that males are permitted to swim in a pool with women who are wearing a swimsuit or bikini even if they averted their gaze. The environment is haraam. Now what if the women were wearing the Islamic style of swimwear - is such a mixed pool still haraam for Muslim males? It goes back to segregation for the sexes in a pool or outside the pool. Some scholars may say the swimwear is too tight so fails for a suitable hijaab also.
Theoretically this is no different from everyday co-existing, but in water instead of air. As usual, both men and women would need to cover their awrah. However, it's practically problematic to do so. The main issues described by fatawa (below) are "temptation" and "uncovered awrah".
I asked about the women's swimwear here: What should a Muslim woman look for in a burkini? I also bought a burkini myself (when I was in Turkey), and tried it on in the pool. Basically, they're either skin tight, or become skin tight when they get wet (as described at AboutIslam). They're also very difficult to swim in, to the point where they might be a drowning hazard. Combining these factors, it seemed rather pointless.
Fatawa are quite strong about this, describing it as a "great evil" (IslamWeb) and that there are "other un-Islamic or unethical factors" (AskImam). Islam Q&A goes further and says its not even permissible if it were just women.