Not sure if this helps. Back in my big boat days. I had 69 commander 31ft. I would buy insect killer at the local hardware store something like "home defense insect killer". On the back label it says to kill spiders it has to be sprayed on contact with them. At night when spiders come out I would walk around the boat and spray entire boat down and dock killing all the spiders. The next morning at daybreak I would wash the boat down and remove all the dried up residue and dead spiders. This method did not affect the gel coat, and it worked very well to keep the spiders off the boat.
I still do this to my house, killing all the spiders at night.
Maybe try a spot under the deck and see if does something to the Anodizing.
Has anyone had good results with peppermint oil products to deter spiders?
I was looking at the off-the-shelf stuff from Star Brite (Spider Away) which, according to the label is about 2% peppermint oil and 97% vinegar and soap... second part of the question would be whether spraying vinegar on /around anodized aluminum is a bad idea sine it is acidic?