Years ago, when I was diving at least part of every trip, I made a discovery that has affected my fishing and led to new and more productive techniques for me.
Sometimes, and more often than you might imagine, the surface water is crystal clear while the water near bottom is murky. I've seen times when this murky, even muddy, layer was more than ten feet thick. In this situation, visibility changes from practically unlimited down to zero. As you might imagine, spearfishing ranges from difficult to impossible. Now, think about the implications for hook and line fishermen.
You might be wondering what causes this phenomenon. Thermoclines are often the culprit in deep water, but strong tides affect clarity too. In warmer water, plankton blooms and clouds of algae affect visibility. Generally speaking, in the Gulf of Mexico, late winter through early spring provides the clearest water.
A powerful and properly tuned depth finder will sometimes indicate murky water. A good color depth finder is easier to interpret than a monochrome machine. I run the gain on my DF higher than most folks and I seldom use auto functions. Thermoclines are generally barely visible as a thin line above the bottom. When a thermocline is strongly visible on your depth finder and there appears to be a lot of "hash" underneath, I expect visibility to be poor. I've predicted this when preparing to dive and, more often than not, I have been right. This is a good reason to have someone who can make deep free dives jump in and take a look before everybody suits up.
Seriously murky water may explain slow bite days. If we have a game plan to counter poor visibility, we are sure to be more successful. Let's develop a plan.
Fish locate your bait or lure with at least three senses; smell, sound, and sight. Different fish depend on ones of these senses more than others but let's just talk grouper here. In very murky water, visibility is often limited to mere inches. In this case, we should respond by using smelly baits, using baits that give off strong vibrations, use vibration/sound producing devices, and doing anything possible to enhance a bait's visibility.
Chumming, in this situation, provides its own Catch-22. A lot of scent in the water may interfere with a fish's ability to find your bait. The fish's olfactory apparatus may be overwhelmed by the chums scent and your bait lies on the bottom, unnoticed. No, I can't prove this happens but I'd give it high probability.
I may give grouper too much credit but I prefer to think that fresh bait is more attractive than long dead, several times refrozen bait. A fresh Spanish sardine, hooked through the eyes and twisted so that the skin and body cavity is broken open has always worked wonders for me. If stinky dead stuff works for you, go for it. Red grouper are far less picky than gags.
One of my favorite responses to murky water is a lively pinfish or small blue runner hooked upside down on a heavy jig head. Hooked through the lips from the top down and out the bottom, the bait fish struggles mightily to right itself. Vibrations/sounds of this struggle often provoke a savage bite. Unfortunately, baitfish hooked this way, tire out very quickly and soon die.
There are several other possible sound/vibration producing techniques you can try that may make your fishing more successful. Bouncing your sinker on the bottom gets a fish's attention. Try replacing the sinker on your "dropper" rig with a piece of chain or a sinker made out of steel rod. This harder material is definitely louder. Japanese long-line rattles attract fish from a long distance. Try rigging one on your line between droppers. I've seen this work.
Hot fluorescent colors are more visible than regular colors. A fluorescent bass lure skirt or a fluorescent tube squid on your baited hook may make the bait easier to see, even in the murkiest water.
Adaptability is the key to consistent catches. Develop a game plan for murky water and you will have fewer days to complain, "They just weren't biting today."