While box turtles are definitely “land” turtles, in the sense that in many ways, their habits and environments more closely resemble that of tortoises or terrapins than of true aquatic turtles, they also love to swim.
Some box turtles will swim several times a day, taking time between swims to dry off and warm up again before going in for another dip.
If the water is warm and sufficiently shallow to allow for resting in place while in the pool, a box turtle might spend quite a bit of time swimming during the warm season!
Of course, since box turtles are “land” turtles, no box turtle will do well in a water-based habitat, and too much moisture exposure can lead to shell rot and illness.
But if you offer your box turtle a sufficiently roomy pool with easy entry/exit that isn’t too deep (water that comes up only as high as mid-shell or lower), it will likely get lots of use!
The most important thing to remember here is that box turtles, like all other reptiles, can’t self-regulate their body temperature. So pool-side safety is a must.
Whether the proffered body of water is a small local stream, a water dish, a kiddie pool or a bathtub, make sure the water is tepid to warmish, and never leave your boxie unsupervised in a water-only environment.
Also, chlorinated pools should always be off limits. Chlorine can be very toxic to box turtles (along with most other beings).