In reply to CurlyStevo:
Bare in mind that the glass/water will have a magnifying effect on the turtle
Agreed this a big problem with reptiles as pets. And while rfuk will happily say this is big enough. What they mean is its the minimum considered ok. however the last thing to do with a reptile is suddenly increase its space as it will display symptoms of stress. As you go larger do it gradually using false walls.
To the op have you got an island or something the turtle can get out of the water if it wants?