Unless all the sides slope, I wouldn't do it. Some tortoises float, but desert tortoises aren't one of them. If they fall in, chances are pretty good they'll drown.
It might be ok, but sloping sides! If they fall off a sharp edge they probably will end up on their back and will drown. Some tortoises (not species, but individual tortoises) have no depth perception and will walk right off the edge of something - the pool, the steps, a cliff, etc.