Swimming in the sea / swimming in the ocean can be great for forming dreadlocks. The sea water dries out the hairs and the salt increases the friction between the hairs which leads to more knotting. If you're able to swim in the sea - I highly recommend it for the for helping out your dreads!
Unfortunately not everyone can swim in the ocean all that often and so will miss out on the benefits - luckily the effects can be recreated at home with a sea salt spray!
On the upside you can quickly and cheaply create a spray that will help with the locking up of loose hairs and the maturation of the dreads. You know exactly what's going in and so exactly what's going onto your dreads. All ages of locks can benefit from the spray - whether they're young new dreads, or older mature dreads.
On the downside leaving too much salt on for too long can dry your hair out too much and if it's left on your scalp can become irritating.
You have little control over which hairs become more knotty and so you can encourage hairs to lock that you might not want to lock... leading to more separating being required. The spray will also only affect the hair that it comes into contact with so will never by itself be a perfect replacement for swimming in the ocean.