At first it was only the Terrapins I had added to my pond that were climbing onto my Lavaiathan, which wasn't a big deal because I could still use a water bucket to pick them up and remove them if need be, but then two of my sea turtles climbed aboard as babies and I can't get them off even after they grew to adult size. I guess they're just stuck there?

Hitting the stuck sea turtle didnt work, I only hit the Lavaiathan, using leads and sea grass didn't work either, though the sea turtles still did turn to face me when i held the sea grass out.