I think there was a thread on just this problem a while back. Might want to search for it because a lot of people offered solutions.
We find that often times spider infestations come in waves because they have hatched and are flying through the air landing on boats. Usually you will see wisps of their parachute threads in the rigging
when this happens. Hitting areas fast and hard with spider spray often gets rid of them quickly.
Other times, we get them from being tied to a dock
or on land for a length of time. Then when we are on the water
again, just killing them as we see them seems to get rid of them after a month.
Of course, getting rid of their food
supply helps, but who doesn't cruise
where gnats and other bugs exist? The good thing is that spiders help with bug, but the bad thing is that they then poop those bugs everywhere and create a mess.
Having a gecko or two on board really works wonders on spiders. I suppose that is going to be a bit more difficult where you are.