This is a fascinating subject and one that has been mentioned quite a few times on the forum.
the legalities of collecting seaweed are well covered and as long as permission is sought and granted - there will be no problem, as long as it is for personal use. This is mentioned in the thread
http://chat.allotment-garden.org/index.php?topic=86170.15We too, took the precaution of contacting the authorities - the council - who were to be quite honest, a little surprised that anyone was asking.
We live on the NE (UK) coast and we do get a lot of seaweed washed up. During the course of a couple of weeks this is pulverised against the shore. It is, quite literally, possible to collect wheelbarrows of the stuff. At certain times, it piles up so high that it becomes a health risk and the council have to bring in contractors to move it a km down the beach away from the seafront.
Anyway I digress. All of our gardening is at present done in pots.. a lot of pots. We regularly collect seaweed now and use it to bulk up compost. At the start of the growing season, we collect pieces of 1-2 inches long, this is really easy to find and it only takes a few minutes to collect as much as we can carry. We lightly rinse it and remake all of our compost and add a little FBB too. The compost is beautiful throughout the whole summer and very rich, well aerated and full of worms despite being next to the sea.
I guess that we introduce approx 15% by volume each spring. It is thoroughly mixed and there is no smell whatsoever.
We have the happiest dahlias, fuschias, etc, etc and veg (lettuce, pakchoi, etc) grows very happily in it. We've just lifted all of our bulbs out of the pots for the summer and we have 25% more each year.. all very happy with hyacinth bulbs the size of turnips!!
We think that it's one of the most under-rated soil conditioners. We wouldnt put it knee-deep on an allotment plot for example because it would not rot down properly for a while and be a little bit smelly, but with a little care and consideration for other people, it is a winner! definitely a great 'no-dig' component
I'd be ever so interested in hearing from other people who have used the raw stuff in this way.