I've not had one in my hands yet, but it seems like there's a new source for battery powered water pistols that look easy to adapt for switch accessibility. It's an AK47 Aqua Fire Water Pistol. It should be quite easy to adapt, roughly following this old "Water Blaster" D.I.Y. Guide.
Try eBay and Google Shopping to track these down whilst you still can. If you'd like for me to adapt one, just get in touch. Link via The Gadget Show.