Well, since it's already filled, you can't let them run around in it and know that the steps/tanning shelf are the way in and out.
Therefore, I'd suggest taking them in with you many times and guiding them to the tanning shelf/steps. I used to do that with my Golden. He'd pretty much relax in my arms and I'd push him to the steps. He'd swim in that direction, hit the steps, and get out. They'll catch on. Just think Pavlov. Repeat, repeat, repeat.
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