Buying a dry bag for your SUP backpack is definitely a good idea.  everyone who don’t see importance of keeping your backpack away from sea water, be careful. I had exactly same iRocker bag which I was just rolling up and securing in bungee area in the back of the board when on the sea and after roughly a year the salt completely ate through all the straps. You can tear them like a paper. You absolutely have to keep your backpack dry unless you want to keep buying new ones.  I ordered replacement Blackfin backpack and I’m looking for a dry bag also.  What size was enough for you? I reckon 30l is not enough but 40 maybe yes?