Why was there a need for Yona to be saved in a miraculous way?

Question

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?

Answer 1

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.

Answer 2

Being trapped inside a fish is a greater miracle, because you can't survive without oxygen, but Yonah survived there without oxygen (see Malbim).