My parents live by the sea in Spain. When they moved in, the garden consisted mostly of builders rubble, sand, and basically no organic matter at all. They have put some compost in but also HUGE quantities of seaweed washed up onto the beach. I don't think they even washed it - they were collecting it by the carload, 10+ sacks a time. (They do water a lot though, as in a LOT, which probably helped to wash out any residual salt.)
Results: beautiful soil, loads of fruit and veggies. They used it for *everything*.
There are also these fibrous round things, not sure what exactly but some sort of seaweed, ranging from about the size of a golfball up to slightly larger than a tennis ball - which they call either "donkey b*****ks" or "donkey doo-doos" (depending on the audience), if that helps you to visualise them.
Ahem. Anyway, I think they wash up seasonally, in the autumn IIRR, in huge quantities, and go on as mulch... As far as they are concerned, seaweed rocks!