I have an YOB Aquamax SUPB. I have always rinsed it after use and I used to hang it in a roof space under a pergola, sometimes covered, sometimes not, which was foolish. The blue covering on the top surface now runs like a blue ink bottle everytime we use it. Legs and bathers get stained in blue. Is there any product I can spray it with without it losing too much grip? I have tried scrubbing it with soap and water but the problem remains.

I would spray some anti-UV varnish.You can always spray some clear anti-skid on top of it afterwards if it goes slippery.
(anti-skid spray cans for pools, decks, stairs...)

Note that I spray anti-UV varnish on everything that stays outside:
stickers on my van, nameplate on my mailbox, most plastics... incredibly effective