One of the ideas is that there should be very few occasions to use written communication underwater. The dive planning, and the pre-trained knowledge of how to operate as a team, should cover the vast majority of situations. I don't do deco diving, but my guess is that if the schedule is not going to be done on the fly, it's been written in the wetnotes before the dive (just as our cave maps are). Pulling wetnotes out to look at them is not a big deal.
When I used a wrist slate in my first cave class (non DIR), I found it to be a nuisance. There was no comfortable place for it on my arm, and the pencil kept coming loose and hanging off me (an entanglement possibility, and aggravating to boot). I did think maybe one would have a place if you were taking down data continuously, like for survey, but they don't hold a lot of data at the size I can write underwater.