Setting up an enclosure for an aquatic turtle and taking care of it properly, before you're able to do that, you need to know what species you're dealing with, and what its natural environment would be like.
There are over 285 species of turtles in the world, and they all have very different behaviors and requirements. The first thing of course that most people thing of when setting up an aquatic enclosure would be a fish tank of some sort, and this is pretty standard when you first purchase a small or young aquatic turtle. Often aquatic turtles, though, grow, and after five or more years, you can end up outgrowing your average size fish tank, and you may need to upgrade into a larger plastic storage bin, or even some type of outdoor enclosure.
When you have an aquatic set up, obviously water quality is very important. You want the water to be of an adequate depth for the turtle to swim, and submerge itself and be able to exercise. Generally we're looking at heating the water. There are water heaters that can be purchased that go inside the tank. You're also looking at, generally, water filtration. Turtles are messy, and it will be very difficult, especially with a larger tank, to be changing the water everyday, which we need to do without a filter.
All aquatic turtles generally still require the ability to come out onto a land area and bask. So this is where they come out, they warm themselves up generally in their natural environment under the sun, but in a captive environment we often provide them with special lighting and heat. So the most common are incandescent bulbs and obviously our UVB producing bulbs. Aquatic turtles should still be provided with a source of ultraviolet light, and a good heat and basking source.
Many aquatic turtles are omnivorous, meaning that they eat both animal and plant matter. Again, you want to look up what your species of turtle is, and make sure you get the diet correct. There are my types of commercial foods for aquatic turtles, they can also be supplemented with things like small fish, crayfish, snails, and aquatic plants.