Spiders...
at the "Top of the Bay" find their way aboard the boats on mooring at our club, so it isn't a matter of climbing the dock lines. They get carried aboard by the wind. I've knocked them off the boat, only to see them walk on the water surface tension and climb back aboard up the hull side.
We use Home Defense spray around the docks and under the toe rail. I've had good luck with the Oceanics "Spider Away" from WM. I've found that dryer sheets don't work for much more than a week.
The most effective way I've dealt with them is to go on "spider patrol" after dark with a flashlight and a can of Raid insect spray. Hit them in their webs and use a wire wheel brush in the morning to clear away the web residue. Look on deck for "spider poop spots" to find out where the buggers are hanging out during the day--come back at night and you'll find a web above these spots.