Haven‘t done Sharkskin as an undergarment (as mentioned, its non-wicking might be uncomfortable, although at 20C you will probably be ok). But for the sake of comparison, I have done the Azores a couple of times in 4th Element Xerotherm/membrane drysuit at 18/19C surface, down to 13C at 35m for an average of 60 mins and was fine. I‘ve also done lakes at 8C - doable for 45-60 mins, but definitely cool after 30mins or so, added a baselayer of ski underwear and felt much better. In your place, I‘d experiment a little.