I trained mine to a wide ramp, initially with walls on both sides to herd them on to it. Once they figured out the routine and realized there was water on the other side of it, they used it with ease...it took about a week of herding them to it and over it, though.
I have a ramp made of two fence board next to each other with little pieces of wood nailed across for traction. They also have cinderblock stairs to the same pool which is about two feet high. They prefer the ramp.
I have another pool with a pallet next to it which acts like a deck. They like that too.