.....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 ). all went well till retrieving the reels/lines back in, which we used for triangulating a referrance from 3 other wrecks so as to build up an underwater tour..... Then one of us lost the slate while reeling in the lines.... ahh well lesson learnt.. (next time im holding the slate.. )