I'm thinking of a low fence, one I can step over with ease, how high can
turtles climb?

Depends on the turtle and the structure of the fence. I would angle
the fence in, towards the pond, and I think you'd have success and
still have it low enough to step over.

Why safe? What eats turtles besides alligators? I can see some critters
getting baby turtles, but it would seem like a real chore as they get
bigger.

I mean 'safe' more in the sense the turtle feels safe. They need
lots of basking time in the sun to dry out spots where interlopers
hide,
and dry out fungal infections, to produce the vitamen that keeps their
shells
healthy and the heat of the sun keeps them moving.
An island is ideal, or a floating log.

k :-)