Here in the UK for site work outside we would typically expect to be using a 110v isolating transformer (if I'm not mistaken, these are 55v-0v-55v, the center tap being used as the earth for the equipment connected to the secondary side of the transformer, with no direct links between any part of the primary or secondary connections).
Given you'll be working with water, the real answer is as low as possible whilst still being feasible. I also don't fully know what the laws are about selling things like this, but I suspect there would be a lot more hoops to jump through if you start connecting them directly/indirectly to a mains derived supply. You certainly shouldn't think about just buying a switch mode power supply brick from some supplier on AliExpress or whatever it's called because these often have one side of the output connected to the input and really that's not acceptable for this kind of environment.
Personally, I would avoid this unless you have absolutely no other choice because it increases the complexity of any kind of certifications and also increases your exposure when it comes to potential claims that could arise as a result of injury or death if the power supply went south and the camera was suddenly live.
