I'm not a boy, obv, but growing up my sister and I spent at least 50% of our leisure time with my male cousins of similar age. We did more explorer-type things...riding bikes/climbing trees/crawling around in the canal/etc. In summer we spent a lot of time in the water and playing with water pistols, but it was always very literal - the intent was to wet the other person, not role-playing violence per se. We all watched tv with guns & gun violence, but never recreated it in play. We recreated violence in the form of wrestling, mostly.

As far as ninja goes, I would hope he has a similar experience. Water pistols are just an efficient way to spray water, and guns don't really factor into play. I have serious reservations about giving children any kind of toy weapon to play with, regardless of gender. While I can't control what he does with other children (once he reaches school age), I can make sure that he understands that real guns are dangerous and violence has consequences.