I take the mesh bag with all my equipment and a dry bag (it's probably one of the most useful things you can own).
The mesh bag has all my equipment (BC, regs, suit, fins, spares etc). I wear my computer on my wrist but if you have a bulky computer, it should go in to your dry bag.
The dry bag contains the most basic essentials and items that should remain dry. For those items that have to stay extra dry, a water-proof case or even ziplock bags are useful.
The rule for boats is take just what you need and make yourself small - you usually don't have that much space.