Years ago, I bought my wife a 250 gallon water pond set up. I got a bunch of creek rock and built a really nice waterfall. It looked great. We put some gold fish in it to control the mosquitoes and other insects. I in the deep south so the summers tend to get quite dry. It wasn't long before I noticed bullfrogs had also made a home in my pond. Then, not long after that, I would walk out to catch a glimpse of a few water snakes that had also took up residence. They would hide and lay among the creek rock. I'm guessing they were attracted to the water source and the fish and frogs which is their main diet. They are harmless, yet most people mistake them for the Copperhead or Cottonmouth. These two are really bad news if you happen to get bit by them. I personally enjoyed the little water snakes, but my wife had a fit. She didn't like them around plus they were eating all her fish. I finally, caught the snakes (2 of them) and relocated them a few miles down the road in a creek.
A few weeks later, I noticed a rat snake laying on the rocks. I figured he probably came up to get a drink. I got him and carried him at the edge of the yard and let him go in the weeds.
Unless you have your water garden in a fenced in area, keep in mind that it will probably be a water source for various critters, especially if you tend to have long dry summers.