If you want to wear the contact lenses to help you own the vision, I suggest you to have the sunglasses outside. The water which is consisted of much bacterium may let your eyes get infected. You should avoid touch the water in your eyes. Thus, you'd better not wear the contact lenses.
You would better not wearing contacts when you playing polo, for the sake that, the water in the swimming pool usually has a lot of hypochlorous acid, it may mixed with the contacts solution which contained in contacts. It will do harm to your eyes and hurt more. Nowadays, there are some swimming goggles which have prescription. I have worn this kind of goggles for years, very useful and convenience, go for a try, why not?
soft contacts usually slip during play, but its really hard for them to fall out. hard contacts usually dont get washed out, but if you blink the wrong way then its over. i play with both, and the thing i do is i make sure that i dont open my eyes underwater. if i have to, i squint, but other than that if i stick my head under to draw a foul or what not i keep my eyes closed. if that doesnt work for you, you could always learn to play without contacts. When i first started, i learned how to play by seeing colors (caps are usually contrasting colors) and just go for a yellow blob (the ball). goggles are not allowed in water polo games so i do not recommend getting prescription goggles for water polo.