Land is something we search for when out at sea, but also something that can destroy our yachts if we bump into it. How close can you get to land?
An easy way to think of land is to ignore the water you see. Now, you will be looking at topography without the water obscuring your view. The land you see is actually just taller points of the ocean floor that managed to perforate the surface of the water.
Since the water is out of the picture, simply visualize the land extending off into the distance. You know how deep the water you are floating in is, so you know how far down the island needs to reach to make it all the way to the oceans floor. If the edge of the land comes in steep, then it is safe to interpret that the topography will continue on its steep path all the way down into the water. If the land comes in nice and gradually, then you can assume that it will also continue to proceed at a slight slope under the water.
Steep land means that it will probably be very deep right up to the waters edge, while slow and gradual land will be shallow far from shore.
With these ideas in mind, you can figure out how safest to navigate around land masses. Steep cliffs will be deep but the landmass can destroy your hull, while gradual shores will be shallow (and provide a bottom that your anchor could possibly reach).