4 of us are going to do the Oslob whale shark snorkel activity in april and I was wondering if weights are needed and how much to be able to make the most of the experience. We will be using cameras sans flash and want to get some underwater shots too.
You are asking how much lead to use when snorkeling/freediving? That is like saying, "how much lead do i need to scuba dive"? The answer to either question depends on your exposure protection, you body type with respect to natural buoyancy and for freediving it is even a little more complicated because you want to have a good idea about your expected depth and you can't offset a mistake by cheating a little with the BC.
For freediving/snorkeling you want to be able to rest comfortably at the surface with your lungs about half full of air. You should not start sinking, even if you exhale. As you dive, you chest will crush, as will any wetsuit, and you will get heavier and heavier the deeper you go.
Freedivers who go to 40 or 60 or more feet, generally try to wear enough lead so that they float to the surface from a depth of 30 feet and will sink below that depth. For "snorkeling" shallow you may want to wear a little more lead so that you are neutral at shallower than 30 feet, especially if you expect your max depth to be 20 ft or something. When you are weighted safely, you will definitely have to "fight" your way down to 12-15 feet - possibly deeper. I suspect your goal will be to swim down and hang in a neutral condition and be stable and motionless. What depth that occurs is up to you, but it is safer to wear less than more lead.
Lastly, the difference 1 extra pound of lead on the belt is surprising. you want to experiment and get it just right for optimal performance and comfort.
This video, (my son diving with sharks) provides a decent example of weighting. Start at about 54 seconds. The diver is swimming down about 25 feet or so and is hanging easily there, maybe floating a little bit up, but holds his body horizontal which creates a lot of drag and prevents the diver from sinking or rising fast, even if the buoyancy is not perfectly neutral.
When he wants to come up, he just kicks into a vertical position and with a gentle kick, he starts a relaxed ascent and the streamlined position and increasing buoyancy associated with the expanding chest and wetsuit, make for a very relaxing, easy, gentle ascent. You wanna try to be similar to this I think.
[video=youtube_share;2sT8Vwyw0ew]http://youtu.be/2sT8Vwyw0ew[/video]