Salt water to us thousands of years ago was almost useless, couldn't drink it, couldn't live on it, but we could get food from it, and that is fish, although it is more demanding for resources collecting fish rather than land animals, it provided our ancestors with an alternate food source. Even today we use not much more than transportation and a food source, we have developed multiple ways of catching enough fish to feed people and are able to make profit from. But this has drawbacks, when catching large groups of fish (especially with nets) they can catch other species and eventually hunt a species to extinction. There are rules in place but governments can never stop everything. But with negatives there also comes a positive, people are building fish farms where they can efficiently breed fish for meat. But even with modern marine farming, illegal fishing has created a big dent in ocean life, with other factors also such as pollution, but if it keeps on going ocean life will dwindle down to a point of no return, but this can be stopped if we support organisations behind fish farming.