hey mariam, first of all, getting ur dreads wet everyday could be damaging as u have to give ample time between washes for drying. especially dreads when mature can take much longer to dry. if they stay constantly damp then u'll invite mold which will make ur hair smell awfull n it is very unhealthy overall.
yes salt water will speed up the whole dreading process. but salt water basically makes ur hair n scalp very dry. i dont think anyone who uses the sea salt spray out here uses it everyday either... i have never used sea salt spray myself. i stay close to the beach, once a while i just go out and have a nice dip n swim a lil. those who use the sea salt spray use it like twice a week i think. another thing, never let the salt water stay in for too long as it will fry ur hair n will get ur scalp really itchy n dry. its very necessary u wash ur hair well after swimming in the sea. plus sea water may even contain unwanted things which u certainly dont want to stay in there.. hope u have good journey ahead and have loadz of fun on the island. peace..