Nice dive, looks like you would’ve had fun!
Re streaming quality, is the display in QGC clearer than the output stream from OBS, or is the issue with the quality coming up from the ROV?
For the former try increasing the stream bitrate (settings in the bottom right of OBS, click stream on the left menubar, up the number to say 4000), or possibly try adding a noise suppression filter if you mostly care about smoothness (click your desktop Source, then Filters (next to Properties, just above the Sources section), click + (above Effect Filters) and add the filter you want. RNNoise is relatively new but apparently works better than Speex (if you don’t have it try to update your OBS - I only got the update this morning).
If the problem is with video from the ROV then a noise filter in OBS might still help, but you also might want to check that you don’t have any scratches on your dome, check the CPU Load (can check at http://192.168.2.2:2770/system while operating) to see if it’s struggling, and possibly check for corrosion on your wires, and look into your gstreamer parameters (if you’ve changed them) to check that there’s no slow or quality-reducing stuff going on. Oh also, might be useful to tune the camera display profile at http://192.168.2.2/camera if you’re having particular colour or brightness/contrast issues.