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.
I believe the ones that you will see commercially available use either a 3 or a 7 pound lead weight, and fairly light line.
Depending on the water you are going into, fairly calm versus fairly current affected, you would want to have a weight heavy enough to make it to the bottom and stay there long enough for you to get a valid reading.
You might want to use rode depth indicators or knots to indicate depth.
I had one for years and found it great for exploring in very shallow areas. The nice thing about a lead line is that you can read the water depth 20 to 40 feet away from the boat. I must admit that it was indispensable in figuring out where the deep water is when you are aground before you try to get free.
Lead lines are easily made but the actual "lead" is probably most easily bought. Traditionally, Leads were 5 to 10 pounds of lead cast into a tapered cylinder. There is a hole in the top of the cylinder so that the lead line can be attached. The other end of the ''lead'' had a hollow that traditionally was filled with tallow so that samples of the bottom material can be brought up. WoodenBoat Magazine sells already cast ones.
The one that I had was made from a 8 or 9 inch length of salvaged bronze 1 1/2" propellor shaft with a hole drilled in one end. I didn''t bother with the tallow pocket. I used a 70 or 80 foot long piece of 1/4" dacron three strand. One end was spliced through the hole in the Lead and the other end had a small eye splice that to would slip around my wrist so that I could not loose the line on an overly ambitious throw.
I made whippings every six feet on center with the first six feet measured from the bottom of the lead (the tallow pocket end). At the first six foot mark I made one whipping (stitched through the line so it would not move) and then at the second made two and so on up to 30 feet (five whippings) and then started over with 1 whipping again with the each of the six foot marks measured to the first of the whippings. I used a tarred marline for the whippings which made them dark brown against the white line. I doubt you can buy tarrd marline anymore so you might have to color the whippings with a laundry marker. (I also used red yarn to make two foot marks for the first 12 feet.)
To use the lead line, you stand on the bow and with one hand spin the the lead in a vertical circle on the end of about two to three feet of line. The rest of the length of the leadline is carefully coiled and held in your other hand. You release the leadline so that it leaves your hand on about a 45 degree upward direction. Prperly thrown the lead enters the water almost vertically and sinks to the bottom with the line almost vertically above. You can actually feel when the lead hits the bottom through the lead line and read the number of marks on the line calling out the marks to the helmsman. You then recoil the line and make your next throw. Some people do not pull the lead off the bottom until they are over the lead so that they make sure that the boat passes over the exact spot where the reading was taken and others like mayself would pull up the lead as fast as I could to take more frequent readings.
You should really practice on the end of a doc and later on the bow of your boat before you really try to do this in ernest. Thrown right you can aim your throw pretty precisely 20 to 40 feet away.
There are tricks to keeping from soaking the deck of the boat with muddy water. (start each throwby twirling slowly and at an angle to the boat and then speed up and bring the twirl to the angle that you want.) If you have to go very far you will wear yourself out after a while.
20 plus years ago I had to deliver a boat back from Charleston, S.C. shortly after Hurricane David and large sections of the intercoastal had shoaled in or shifted. There was one two mile section that we had to find our way and at the end of that hour or so my arms and shoulders were really warn out.
Belated thanks for your referral. Yes, we do indeed have leadline weights, both in lead and in bronze. They''re the same size, so the 7lb LEAD lead translates to about a 5 lb BRONZE lead (if you get my drift....)
The 7/5-pounder is the smallest, but larger ones are available for them as wants ''em.