My SeaDek has been holding up great so far in the FlaDa sun (installed 12/1/2020).
Fish blood or bait does not make any stains as long as you clean up the mess and hit it with the salt water wash-down....which you would do anyway even it there was no deck covering.
The only issue is it's tendency to get cut and sliced up. You need to be careful about dropping knives or gaffs onto the deck, and even when landing flopping fish. I have a couple of "souvenirs" in my deck which were made from Grouper gill plates slicing the deck.
My biggest mistake was storing my crab trap on the deck during the summer off-season. Last year an Iguana got into my crab trap, got stuck and eventually died in there. He must have made quite a struggle trying to get out and his claws left their mark on the SeaDek.
Luckily, my Son came to go fishing on my boat during the summer and he was the one who had to dispose of the smelly mess, not me...