Best place I can war at has been the same for 5 years; me and my friend's backyards. My favorite place I've played any wargame at however, would have to be the
NoLimits Extreme Action Park, where I went to play airsoft several years ago. Surprisingly, most of the cover is spaced well enough to be good for water wars it seems. The only thing missing there would be hoses lol.
I've had wars at Boulan Park and Firefighter's Park in Troy but neither of them were any good. There's not enough cover in the woods of Firefighter's Park and Boulan Park was where that one incident occured. (Long story short, people there look down upon teens and young adults playing with water guns on a playground and the playground is the only area with any cover to play at.)
Overall, not a whole lot of inspiration I can give here. I need to go around looking for wooded areas where no one will bother us, but there needs to be sufficient cover/concealment and having trails would be nice. I haven't tried any water wars in the woods and have had some particularly difficult games with airsoft in such environments; they're difficult to play in, you need full camo unless you want to get scraped by twigs/plants, etc.