Makes sense, not sure why I assumed the ROV had to make the full trip alone - I’ve probably been reading too much about fibre optic tethers and the like lately…
It’s a bit hard to see that from the image, but I understand your point.
As a side note, I’d recommend moving that large black component out of the view of the camera if possible, because it’s likely making it significantly harder for the camera to adjust its exposure correctly. If that’s not possible then I’d suggest you try adjusting the Camera Settings through the web interface, and set up a profile that works well at your operating depths 
For now, I cropped out the black part so I could adjust the photo to see it more clearly. We’re considering adding some similar processing to the QGC live display to improve visibility, but if there’s a constant large black portion of the image that kind of processing likely wouldn’t help very much.
This sounds like it’s perhaps mostly a tether management issue. Ideally the tether shouldn’t get close enough to the marine growth to get caught on it, so your ROV should either stay above the level of the marine growth, stay off to the side, or the tether should be pulled more taught when passing over marine growth so that it can’t droop back down afterwards and catch on it. That’s understandably easier to monitor if you have an additional camera pointing back towards your tether, but it’s perhaps also possible to turn around and check the tether is free when you pass over something spiky.
If you add a buoyancy block to the tether that could lift it higher behind the ROV, but would also provide another component of the tether for things to get stuck on. My intuition is that if you add enough foam to make a significant difference to the tether orientation while the ROV is in motion then that will also have a non-negligible effect on the ROV balancing, which you may need to compensate for with weight distributed towards the back. I imagine the slim tether would have less of that because it wouldn’t require as much foam to lift a bit, but you can always do some tests with your two different tether types, and different foam amounts and distances from the ROV, and see how much of an effect each option has