How to make and use a Leadline
I used to carry a home made one and I swear I saw a better one for sale.
A lead weight with wax exposed at the bottom may show the bottom conditions too.
Then 3 strand line with anchor rode tags installed would complete the picture.
As often as I used it in the last few decades (never) it''s in the basement now I think. You can always just make one up from something heavy. A instant measurement of a fathom is the reach between your hands extended to the side.