Tom, I recommend a faster, more efficient way of mapping your pond depths than string & weight: A 12 or 14 foot bamboo cane poll. Mark off the lengths every, say, 2 feet, then poke it all around & record measures. As a bonus, you can grind it in to the bottom to check on composition (mud, sand, rock, etc).
Learned this from Overton's when they came out to deploy artificial habitat. And, for those of us who remember our first fishing "rod" as a cane pole, it feels good, too.