Lines of bubbles can also mark current shear points, which warns of a change in water depth. Going around Curves In channels, the current flows faster around the outer bends of curves. This usually results in deeper water on the outside of most bends, as long as the channel is mud. With hard rock there will rarely be enough scouring action. Watch the Critters Animals, especially birds, often give sign of shallow water.
Some birds walk in the shallows, picking up tasty morsels as they go.
Obviously, if the water is shallow enough for them to walk, it's going to be a problem for most boats. As we've said before it is always best to avoid the use of nav markers unless you know for certain they are properly placed. But as you cruise farther afield, nav markers tend to be less reliable. Keep in mind that the buoyage systems are different between the inland waters of the US and offshore. They change from country to country as well.