.....When I did mine I used a slate and wrote down all the headings/distances/depths so that I had all the information I needed from one dive. A tape will work fine but arm spans are nearly as accurate and fin kicks will get you within a few feet. How many to a 100 feet and you've got a pretty good rule of thumb. Then it's just a matter of putting all of ot on dry paper.
This is the way my buddy and myself (he's doing the DM too) did things this morning mapping a new wreck (new, as in only a month or so old, not a newly discovered Galleon or anything so exotic

