Humans with all their dive gear on are positively buoyant. This is an issue for SCUBA, where the whole point is to go down! Weights are an essential, but costly, part of any divers kit; which is why I decided to make my own!

What you need for this project

Sewing Machine. The beefier the better. You are going to be going over lots of seams, again and again, so you need a strong, consistent stitch.
Scale. A kitchen scale should work out well, that's what I used.
Lead shot. The quantity depends on how much weight you need. For me, I dive cold water (Monterey) in a 7mm wetsuit with a full hood and gloves, so I made 25 pounds.
Fabric. I found duck canvas to be the best option. It's relatively cheap, sews well, is strong, and won't degrade quickly in the water. Get more than you think you need, I would estimate I got about 2 yards and had plenty left over for other projects. It is a good material to keep on hand, I love prototyping with it.
Patience

Step 1: Get Your Materials!

Assuming you already have the sewing machine and scale, getting materials is a simple task. I went to the local fabric store, for me, it was Joanne's, and looked around until I found a good fabric. This is the one I used.

I got my lead shot off of Amazon. I feel bad for my mail carrier who had to deliver this 25lb package (thank god for Prime). This is the one I used. If you didn't want to mail order for whatever reason, your local gun/ ammo shop will likely sell it as its main use is in shotgun shells.

Step 2: Weigh the Lead Shot

I knew this project would take a while, so I weighed all the shot into ziplock bags. I found it easier to work out of a small bowl than the deep bag, so I dumped a small amount into the bowl whenever I was going to work. I then used a small medicine scoop to put the weight into ziplock bags on the scale.

Step 3: Begin Sewing!

I started by estimating how big I wanted the pouch to be and made a little pouch that was larger than that. This was my test pouch. I would then put a quantity of weight in it, and figured out how large of a pouch would actually fit that amount of weight. I then laid out all of the cuts I would need to make, cut the fabric with a set of trauma sheers, and stitched most of the bags before filling them. I stitched with the waxy side of the fabric out so when I was finished I could invert it and there wouldn't be external seams. The final seams were the hardest; I sewed all except the last 1/2 or 3/4 inches and funneled the lead shot in. From there, I experimented with different ways to hold back the shot inside so I could finish sewing. I tried clamping pieces of wood over it, tack stitching by hand, and many other methods but the best one I found was stapling. As stupid as it sounds, they worked well and were easy to pull out after. After you finish sewing, be sure to label each bag with the weight. I used a white paint pen that I have for welding and it seemed to work well on the fabric.