I used to keep turtles.
I'd give them more water than that. give them some deep water with some room so they can swim around (yes, even those little guys can swim deep if they want), but definitely give them some light. it doesn't have to be all day, but i'd say at least for a good 8-10 hours a day if possible. Make sure the light isn't TOO hot.
If you have the deep water remember that they need lots of platorms and whatnot that they can easily access to remove themselves from the water. Turtles spend a lot of their time out of the water.
Did you catch these in the wild or buy them at a pet store? Do they have little reddish/orangeish stripes on the sides of their head?