Well, putting the whether you are allowed to take it or not to one side. I collect a big IKEA bag-full on my weekly visits if it's been washed up by the tide.
I don't wash it. I just try to spread it fairly systematically around the garden. I mulched the asparagus with it last week. It rots down pretty quickly, especially during the winter. I have no way of knowing for sure if it's any more beneficial than, say horse manure, but I've been doing this for the past 12 years so I'm pretty sure it must have had some effect (hope so, because sometimes the bags have been pretty heavy!)