This might kind of go under #5, but you need plenty of cover and places to hide. Water warfare doesn't work well as a pure shooting game, and you need a location that will support good-quality fighting. Cedar Hill was one of my favorites due to the mix of open and thick areas, as well as interesting features like the buildings and the pond. St. John's Woods was excellent as well. The worst places are the wide-open ones like, well pretty much any forest area in the winter.
Don't forget that it depends on the type of war as well. A big event with people coming from all over should be in some big easy-to-find park, while a localized competitive war can be pretty much anywhere as long as it's good for fighting and isn't too dangerous, haha.