R/C Water Cannon
This gadget really appeals to my somewhat childish nature, the Radio-controlled Water Cannon.
The remote control water cannon works like the radio-controlled drinks tray (i.e. you can sail it around your pool) but instead of serving your guests drinks you can give them a soaking instead. The water nozzle is adjustable and has a range of about 10′ so ambushing people walking past the pool is a distinct possibility .
You can get the R/C water cannon from Frontgate, found via Uber Review thanks Tiago.