And also, why go swimming in the ocean when there is an internal lake with waterfall and peaceful water (i.e. no undertow) to swim in? one thing you have to realize when stranded on an island is how precious your health is, and how not to take unnecessary chances. In the ocean you can fall prey to currents, sharks, jellyfish, etc.
If I was stranded I would only use the ocean as a resource for food, and would only bathe or recreationally swim in it if there was no other option. The losties need to realize just how fragile human beings are.