Turtles need to get out of the water in order
to bask. They need to raise their body temperature,
it's the only method they have, and they need to dry
out some body parts to keep healthy.
So your turtle would need to get in and out of the
water.
We have an island in our pond that the turtles liked.
But they could also get out of the pond and go
walk about the yard. A securely fenced yard is a must
as turtles have wanderlust at times.

As for your koi, I never had a problem with fish health
and turtles in the pond.

k :-)