You can install the OceanWave launcher to replace the default home screen on Android with a big "OceanWave" button that simply relaunches the Ocean app. It hides all the other apps.
To install the OceanWave launcher, tap the cog in the bottom left corner of the keypad screen. Enter your credentials and pick "Install Launcher" from the menu. Once it's installed, you'll need to tap the "home" button and tell Android to use the OceanWave Launcher "always" to handle the home button action. To uninstall the OceanWave launcher, follow the same steps but choose "Remove Launcher".
Technically, the launcher doesn't prevent very clever patients from installing apps or surfing the web but it will make it quite difficult. The most important advice for managing tablets in your waiting room is always to keep your personal data off them: no Google Play account information, no credit cards, etc.