Well, I don't have any scientific evidence to back up my doings, but I've been bringing back bags of washed up seaweed, bags of shells (plus the shells of the mussels I collect for me and the cat), and a bottle of seawater almost every week for the garden for the past ten years or so.
The seaweed gets spread around as a mulch. (I do eat some of the best stuff and dry some too.) The shells get scattered - crushed eventually as they get worked in - and I've always presumed they give a kind of slow release lime to what is a naturally very acidic soil. (I collect cuttlefish bones too).
The seawater gets diluted 1:10 and watered over the garden with a watering can fairly systematically, but initially it's for the asparagus and carrots...
I like to think it's doing some good, but it definitely doesn't do any harm!