I live near the west coast of Ireland, I mulch all my vacant beds (except those intended for root crops) with fresh sea weed.
I have never washed the sea weed and its never been a problem.
The mulch goes down Oct/Nov, I also add autumn leaves to the mulch just to keep the soil covered. I did a quick check just now, sea weed has disintegrated but the leaves are very much intact, I will rake these off before using the beds. The surface soil is alive with worms and I expect lots of other life forms I can not see.
Very good article on seaweed here >> http://journeytoforever.org/farm_library/seaweed.html
