There's not much you can do if it's been in the water except remove the battery, dismantle the phone as far as possible and let it dry thoroughly. Chances are it's wrecked, but worth a try.
If the phone was provided by your cellphone supplier, contact them as they may supply a new one free.
Otherwise, when you're sure it's as dry as possible and it still won't work, look for the sort of small shop that repairs and sells phones.
Maybe they can fix it. Maybe they will give you something for it if they can use some of the parts. Maybe a reduction on a new phone.