Halcyon have taken the dive slate to a new level with their wetnotes system.
It looks like a small ring binder but it's waterproof and is small enough to fit into a drysuit leg, or BCD pocket. The clear plastic storage pocket on the back is a great place to store written dive tables, or anything that needs to be referred to during a dive such as fish ID tables. It also has a velcro divider that can be used to mark your page.
The main part of the wetnotes comprises of a small renewable ring bound set of waterproof pages made up of 25 sheets of double sided waxed paper to use for all your notes, diagrams, and communication etc.
Apart from being a place to store 2 or 3 pencils (best to use non compressible types, eg recycled rubber), the storage compartments behind the front cover are often used to store essential cable ties, that can be used to make that all important quick fix to a broken piece of kit or bolt snap tie. Divers/Instructors will also often carry a small adjustable wrench or tool depnding on their reg type.