Water depth is very important, especially when navigating in a sailboat with a keel hanging low below you. Charts take great effor to dictate and describe the depth of the sea floor to us, usually with little numbers that are provided by someone sounding that specific point in recent history. In popular areas, soundings are taken often and charts are peppered with little numbers confirming that you will have enough water under your keel to safely pass over the bottom. In areas that are less popular, there will be less soundings taken and less numbers on your chart.
In these cases, your guiding feature on the chart will be the depth contour line. This line is drawn to demarcate where a drastic change in depth occurs. It is equivalent to showing you where a ditch would lie in a field. If you find yourself in less popular waters, you may need to rely on these depth contours to safely navigate your way through the waterway.
If your sailing area is even more remote than that, and the waters aren't even charted, you may need to rely on yet another trick to finding your safe passage.: guessing.
This may sound like a really dumb idea, as you are in a remote area and if you guess wrong, you could possibly loose your yacht, but there is some thought that goes into it.
The ideal would be to anchor the yacht where you are and run ahead in the dinghy to take soundings. These are your own depth measurements where you can see exactly how deep the water is in varous parts of the river. As you move along, you can get a feel for what the bottom is shaped like and that will allow you to navigate the waters much more safely.
If you are not able to anchor and investigate with your dinghy, for whatever reason, there is a less ideal method that you can use, and that is to guess the water depths based on the shore lines. Waterways do not exist as special areas of the land that are devoted to the passage of water. Waterways are merely areas of the land that dip below sea level or the local water table. The contours of the land will exist regardless of the presence or absence of the water that flows over it.
If you see flat lands that lead into the water, you can assume that the land continues on that same angle out into the water. This means that it will be shallow near shore as well as far from shore. The water will very slowly and gradually become deeper as it distances itself from the shoreline. On the contrary, if you see a very steep shoreline, you can assume that it is also very deep right up to the shore.
Flat lands with rivers tend to have rather shallow waterways. Fjords, on the other hand, have a shoreline that could be confused with a cliff! Fjords are cut out by glaciers, and will be exceedingly deep right up to the cliff wall. When I was younger, I visited Milford Sound in New Zealand. I was astonished that the water depth was hundreds of feet deep right up to the cliffs face!
If you are sailing near a coast or shore, you can always judge the slope and height of the land and imagine the mirror image continueing into the water. If you see a flat shore, it will be flat and shallow. If you see a hill leading to a beach, it will be deep enough as you approach the shore, but will get shallow as you near the land. If you see a mountain popping out of the ocean, you can probably sail right up to the edge of the land without much fear of bumping something with your keel.
While these are merely guesses as to the depths beneath the waves, the best tool to have should never be overlooked: good charts and a depth sounder. If you can get good charts, they will tell you about the contours of the bottom and allow you to safely navigate your way through the waters. If you can't get good charts, be cautious and take your own soundings before entering a waterway. If you can't get your own soundings, then guess based on the shores edge.