[IL] Mermaid Swimming through the ground

I have a mermaid and when he swims his tail is invisible. And and one point he started swimming through the ground. He could not interact with any of the objects but would just swim through everything. I eventually fixed it by having him go out to the ocean and then come back to dry land and we got his legs back. But I don't know how he started swimming through the ground in the first place. It might have to do with placing the boats on dry land... I have a video of it but it wont upload.