Samsung DeX is a desktop environment used in combination with a larger monitor when docking a newer Samsung phone. The use of DeX is optional and you can opt out from the mode by selecting "Switch to screen mirroring" on your phone.
How to enable full-screen for the Blueye app
1. Go to Settings
2. Select "Samsung DeX"
3. Select DeX Labs
4. Enable "Force apps to resize". Next time you launch the app it will automatically go into full-screen