The rule of thirds is a useful planning tool for dives of certain characteristics. For other dives, it is the wrong tool for the dive. For most recreational dives, it is the wrong choice.to deviate from the Law of Thirds.
I said I do not carry a spare mask while I dive. I don't. I do, however, have one with me at all dive sites. I once sat on one, and it did not fare well. I am pretty sure I won't sit on it during a dive.Spare mask: the only reason I carry one is because people have broken mine before getting into the water by falling into me or while I am helping them gear up. The majority of the time I end up giving my spare mask to someone who doesn’t have a spare mask strap.