There have been lots of good tips shared already. Here’s what I might add:
If you’re new to flying an ROV, I recommend you pick up a copy of the “ROV Manual” by Bob Christ. The latest issue is available for purchase on Amazon but an electronic version of the older issue can be found floating around the internet. Well worth the time spent reading.
As you have already learned, using visual clues for navigating in poor visibility is really the key. Such as pilings, jetties, etc. But if none exist, you can always add your own using a small weight, bright colored line and a small float. Follow the line from the surface to the bottom, fly in one direction, turn 180 degrees and return to the bottom of the line. Repeat in different directions until you’ve covered that area. Then move the marker and repeat.
Of course the best solution for poor visibility is to use sonar. A good forward scanning / imaging sonar isn’t cheap, but if you plan on using your ROV for work in poor visibility, it is a worthwhile investment.