What's the deal with Yona being swallowed by a fish (2:1)? Couldn't the same thing (i.e. him realizing his mistake) be achieved if he landed on an island?
Being in a cramped fish, which felt like a grave, caused Yona to turn to God in prayer [and repentance]. Source: Rashi to 2:1 and :3.
Being trapped inside a fish is a greater miracle, because you can't survive without oxygen, but Yonah survived there without oxygen (see Malbim).
Take a step back and look at the message of the entire book. G-d tells Jonah, "go rebuke Nineveh." Jonah doesn't want to, because he's concerned with his reputation as a prophet; because how it will work out with history (Nineveh is the capital of Assyria, which will later attack Israel; and besides, it will make Israel look bad for not responding to rebuke). G-d's retort is to have the entire natural world turn topsy-turvy on Jonah -- storm, fish, gourd, you name it -- to prove a point. If you try making all sorts of calculations, even for the best of reasons, to defy the will of G-d, then the world stops functioning as we know it.