This has worked for me on the Island and Sapphire princess. It may or may not work on other ships.
Connect your HDMI device to the HDMI port on the left side of the TV. Use the open HDMI above the ethernet port that the OceanView box uses.
Using the remote, launch one of the live TV channels. The bridge camera or the ship information channel usually work, but you may have to try one of the live TV channels (e.g. MSNBC).
Reach under the middle of the TV to find the on-TV directional jogger. Press in once to open the circular four-way menu, then toggle left and select the input selector. A menu will open on the right side of the TV. There should be a red dot next to "Live TV" (if the dot is next to HDMI, you didn't do step two correctly). Toggle down to HDMI and select it. This should successfully switch the input to your HDMI device.