Do you mean as in what type of fish and bottom life you see?
There are those recreational divers who do help, but most belong to a dive op dedicated to such. I'm sure you can find at least one government, aquarium, and/or scientific dive op who are probably looking for many volunteer divers. Best to contact one in your local area if you really want to help.
There's specific methods and techniques to doing counts for a population survey for both fish and bottom life.
Fish are especially tricky, because if you stay in an area for too long, you will attract some fish while scaring others. It's easy for the presence of a noise making scuba apparatus to bias the data.
Fish transects in my area for PISCO and UCSC sci diving ops require constant movement along a straight line and following a roughly constant depth contour while counting every fish you see. If you don't keep a constant pace, there's a good chance you won't get an accurate count, because fish can hear you even before you see them. If you can't keep an accurate count during your pace, then the data's not accurate.
It's very task loading and not fun at all.
I'm sure other ops follow a similar method.
Substrate and invert surveys are a bit easier, but still very task loading orientated. Unless you plan on making a career or are just stoked about the dirty field work, I would look into volunteering with an aquarium as your first choice.
*PISCO = Partnership for Interdisciplinary Studies of Coastal Oceans
West Coast based scientific dive op. Lead by researchers from multiple university campuses.
