The sailor in the picture is holding a sounding lead. This was used to find how deep the water was. Why did the pilot need to know that?

Here is a drawing of one of the weights found on the Mary Rose.

There is a hole in the top of the weight for the line to be strung through. At the bottom is a small hole that would have held a piece of sticky tallow (animal fat).
Can you guess what this was used for?