The white fungus. Yes, you are on the right
track, you need to have a way for the turtles to
get completely out of the water.

In my pond we built an island. We used aged
builder's brick and put a round, oil filter shaped
planter on top. We cut some slits in the side of
the planter and filled it with aquatic soil.
.. We put creeping jenny around the
edge as it grows over and helps the turtles
climb up. Before that happened we used some
branches to make a ramp of sorts for them.

It will probably take some experimenting to see
how to get something workable for you. But you
are right they need to get all the way out of the
water to bask and dry out all their nooks and
crannies.

k :-)