Few have attempted filming slow motion underwater and the results are usually underwhelming due to the amount of light needed. The light kits alone can make the undersea wildlife exit in a fast way from apparent danger unless they lack eyes or photo sensors. However with a little ingenuity and keeping frame rates slow but not overly so you can get very good looking footage under the ocean with what amounts to be an entry package.
Chase Darnell did a review for Dive Photo Guide where the Sony RX100 IV is used with the aid of a Fantasea FRX100 IV Underwater Housing. As a diver the review is geared for underwater shooting which is not common when it comes to slow motion. He goes over the pros and cons of the kit and much more.