Have you ever thought what else is in the sea water you like to use?
In a lot of poorer countries, they channel their sewerage directly into the sea.
From there it dilutes and wafts all around the world.
There are millions of fish doing the same thing, and you want to wash your dreads on that?
For the softest of locks, a final rinse with a light beer is beneficial, you can check the quality at the same time as well :-)