I have been very curious about this too and have done lots of research. The trace minerals are VERY dilute in ocean water, you have to use a lot. Some companies are using proprietary methods and locations to concentrate these into much less dilute products, although in the end they still have a high percentage sodium chloride. The most concentrated and best value product I so far have found to be SEA-90, with a price similar to Azomite per 44 pound bag or so. SO, it makes for an interesting comparison of their Certificate of Analysis, which can be hard to find from many companies, but I did find some info from Sea-Agri (SEA-90).
I don't remember the exact details, but if I remember correctly, I came to this conclusion: Azomite appeared to be much better rounded in most of the minerals, some which were extremely low in SEA-90, and on average was 2x or more better value on mineral content than SEA-90. Therefore I concluded since I would have to ship tons and tons of sea water or spend 2x+ the amount of money on sea mineral products, I might as well just buy good ol azomite or other rock dust. Maybe sea minerals can fill some gaps, but, yeah