most important instrument is the depth sounder, but I doubt you don't know this now
It really is.
And, the most important thing to know about charts and chart-plotters when cruising the coast of the Gulf of Mexico is to remember that these areas get chewed up by major hurricanes on a regular basis. You see the damage they do on the land. What is not so apparent, is how they change the sea floor, and it often takes a little while for the charts to catch up.
Sand really gets moved around by them. A good example. Look at most charts for the pass between east and west Ship Island in the Mississippi sound. You used to be able to walk between those two islands before Katrina. Most charts still say you can. But, it's 10 to 15 feet deep between them now.
And, all that sand went somewhere.
Another place to avoid is the Cape Romano Shoals. I was cruising by them a few years ago, in what the chart said was plenty of water, and even the depth finder said was 10 feet. When my wife shook me and pointed ahead of the boat. A sand bar, and we were at least seven miles from shore. I looked to the left and right and realized there were bars all around us. I backtracked using the bread crumb trail on the GPS, went out 15 miles and then went around them.
But, it took a long time for my blood pressure to go down.