I tried a new strategy for creating a large lake/exhibit thing for a basilosaurus thing yesterday, and it seemed to work well enough. I made a large fairly deep crater sort of lake, then raised a small part in the centre and put a large water pump on it, and placed 2 of those high water pumps around the edge of the exhibit on small ledges sticking out near the top but not right at the top so they didn't overflow, and one or two large pumps on ledges as well. Then you just wait for it to sloooowly fill up lol. Otherwise i sometimes make a sort of waterfall thing flowing into a small lake and wait for that to fill, mostly just put a few water pumps around the edges of the lake and wait for them to fill it.