Swimming means to simply drop into the ocean without anything to keep you afloat except yourself. There is no 'Swim' button; you automatically float. Many noobs do this because it's faster than a logboat and they don't have an adze for making a raft.
Of course, swimming is not without its drawbacks.
- You will slowly deplete your Lungs and eventually your Health, causing you to die.
- When you arrive at 20 Health, you will automatically Dive and you cannot swim again.
- Pirates will see you as an easy target. They may allow you to come aboard your ship, then kill you for your valuables.
To Dive is to simply hit "Dive" and just sink. There is no drawback of course, unless you are willing to count that your Lungs and Health deplete at five times their normal rate.
The only reason you would do this is if you dropped a Legendary Fish, tool, a lot of food, or a valuable metal into the ocean by accident, or a teammate died in water and wants their things back.
Later, ther may be other things to dive for, such as spearing fish, going into underwater caves, or looking at coral. There could also be a thing that allows you to breathe more easily while diving, such as a wooden tube used as a snorkel.