For years I (Galen) avoided using a snorkeling weight belt. I knew it made diving easier, but I did not want the extra weight in our luggage and the hassle of more gear. But I gave in, and I now love using it.
The only reason to use a snorkeling weight belt is if you like to freedive down to get closer to things, for pictures and to see under ledges. The weight helps you do that with less physical effort, so you don't run out of breath as easy. And once you are down, it helps you stay at depth without having to kick as much, so you don't scare fish away. When I first started using the weights I was pretty surprised how much less effort I had to use, and now I don't want to go without it.
Please make sure and read the safety information below about freediving before trying a weight belt.
There are two snorkeling weight belt material options, nylon webbing, or rubber. They are all 2" wide.
The nylon webbing belt is cheaper, weighs less in the luggage, and is easier to find. Most tank divers use them.
The rubber weight belt is mostly used by spear fisherman and freedivers. The benefit of rubber is that as you get deeper, your wetsuit and your body will compress, and the rubber belt will adjust for this and not slip up your body as easily as a nylon belt.
I bought and use a nylon belt because it was lighter weight, cheaper, and because I don't normally wear a wetsuit, so the benefits of the rubber belt are not as great. But my nylon belt did slip up my body when diving a fair amount. So I will likely try a rubber belt in the future.
There are a few buckle options. No matter what you get, make certain it is a type designed to be able to release instantly for safety.
There are stainless steel metal buckles, or nylon plastic buckles. Most of these are a cam lock clamping style. But you can also get rubber belts with holes and a metal buckle like this one. I prefer the cam style because they are infinitely adjustable, as compared to only where there are holes in the belt. I chose plastic to save weight and to make it easier on my other gear in the bag.
The weights are lead and fit a 2" belt. You can buy them in a variety of sizes, combining them to add up to your requirements (see my weight suggestions below). Although most are 2 lbs and up, 1 lb weights are available which is what I use. I like the weights balanced on my hips and not on my back/kidneys.
If you get a nylon belt, you may want to purchase coated weights so they don't slip around as much on the belt. You can also stop weights from moving with an accessory clip like the d-rings I use below.
There are also weight bags (soft weights), but I have not tested those.
I often attach things to my snorkeling weight belt, like my camera, or shoes. There are a variety of attachment methods available for weight belts. I use these d-rings that hold my weights in place and provide a place to attach things.
This is a complex question. It will change based on your weight, body fat level, if you are wearing a wetsuit, and how deep you dive. Keep in mind that the farther down you go, the less buoyant you will become because of water pressure. You may be positively buoyant at 20 feet, but neutral at 30 feet (meaning you won't rise to the surface without kicking).
I like to stay very buoyant for safety. If I black out for some reason I want to come floating to the surface. So I don't use nearly as much weight as most divers and freedivers/spearfishers. I am fairly slender, 150 lbs, without much fat. Without a wetsuit 3 lbs feels perfect to me. It gives me some assistance without feeling too heavy when swimming around. I can use 4 lbs when I am wearing a very thin neoprene top, or when I have my flip flops attached to my belt that are buoyant.
You will find online sources for figuring out how much weight to use, but they are nearly all for tank divers or spearfishers. For a snorkeling weight belt, it's not as clear cut. You will just have to get in the water and test different weights to figure out what you like. I would suggest only doing this with a partner who can dive, and err on the buoyant side. Also keep in mind that less weight is more comfortable when snorkeling and not diving (and less luggage weight).
You can rent dive belts at nearly any dive shop. So renting instead of buying and lugging the weights around in your luggage is certainly a viable option. But if the amount of weight is not too great like my belt, then taking it with you while you travel is not too much of a chore. Note that airport security will very likely want to open up your luggage with dive weights in them (the x-ray machines don't like lead). So keep that in mind when traveling with weights. You may get delayed.
Snorkeling has its risks, and freediving introduces a much higher level of risk, particularly doing it in ignorance. If you are going to dive you should take a class to learn all the risks and how to do it as safely as possible. If you are surprised to learn that many freedivers black out near the surface, or even 20 seconds after surfacing and taking a breath, then you should take a class.
You want to stay positively bouyant (don't use too much weight), and you need to use a safety buckle that can be instantly released dropping the belt if you get in trouble.
For how I dive, I feel using a weight belt is actually safer than not using one, because I use so much less energy, and so the risk of running out of breath is less. But I never push things. I don't stay down long, and I return to the surface quickly. Most of my dives take less than 30 seconds. I am just going down for quick look or a picture. Sometimes I will hang out waiting for a creature to come out, but not for long. And I rarely go deeper than 20 feet, the average being 10 feet.