To me there is a difference between a water or nerf gun, and a plastic toy gun. In my opinion, playing with a water squirter is like playing in the sprinklers or something, the fun is the water not the "shooting". Nerf guns are also more like playing dodge ball or something. I don't know if that really makes sense, but to me that is very different to a plastic toy gun that serves no purpose other than to imitate violence. I don't really see nerf toys or water as violent play, it's just fun for them to dodge the water or the toy, or to get soaking wet, etc. I don't know if that makes any sense at all, and I don't really know how to explain my feelings on it, but I also don't feel like I would need to explain it to anyone. They are my kids so it's up to me what they play with whether it makes sense to other people or not! You can also get water squirters and nerf guns that don't actually resemble guns. My daughter has a number of cylindrical water squirters that look nothing like guns, and if she ever shows an interest then we would get her something similar nerf style, but my kids certainly won't ever have a plastic gun that serves no purpose other than to allow them to imitate hurting someone. I don't know of anyone that has a plastic gun for their kids to play with so I don't think it will ever be an issue with my kids because I can't imagine where they would even see one.
