Why do you need a surface interval? At 20 ft you have infinite NDL, so your gas supply (and thermal comfort) is what is limiting. If you get tired (cold), rest then. If you are not tired, keep going. If you are cold, get out of the water. If your goal is to increase your dive count, then just put the dives in your logbook (jk) and don't bother doing them...you are cheating either way, so you may as well save gas and time.Also, is there an issue floating on my back during the surface interval. Wading to shore and taking off/putting on gear is a bit time consuming, I'm estimating at that at 20ft a 10 - 20 min surface interval is what I will need between dives and enough time to rest.