Sure, it's not ideal... but these turts have been living in a 50 gal (only filled 1/2way with bricks taking up a good portion of that) for 20 years now... so the 30gal(ish, took about 5-6 buckets to fill) of swimming space in the pond is more than they're used to at this point.. I think they'll manage for now.
I'll also add that water level much higher than it's at, having to raise the "basking spot" and they have escaped (the painted is a little escape artist) I may build something a little better for a platform/ramp though so that it doesn't take up so much of the swimming space when I get a moment, but for now they seem quite content with what they have. I was planning on having the "basking spot" on the left side of the pond where its raised up, and then the other side I'd leave so it's partially submerged so they could have a spot to chill but still hang out in the water. then the center portion where it dips down and creates a pool of course I'd leave open for swimming space.