Yes, the ocean is the setting for this little fish tale, but it's also a pretty important symbol in the movie, too. This big blue force of nature is pulling double duty.
Right out of the gate, we learn that the ocean can be a terrifying place. With Coral's death, the children watching will not only cry at the thought of losing a beloved parent (Coral vs. Bambi's Mom: a debate for the ages) but also they'll learn that tragedy is always lurking around every corner of the deep blue sea.
This view of his watery home is what informs Marlin's every moment with his son:
MARLIN: Now, what's the one thing we have to remember about the ocean?
NEMO: It's not safe.
MARLIN: That's my boy. So, first we check to see that the coast is clear. We go out and back in. And then we go out, and back in. And then one more time—out and back in. And sometimes, if you wanna do it four times…
Okay, so the ocean's a horrific thunder dome where fish must live in constant fear, right?
Not quite. In fact, it's clear from the first few moments of the movie that water is also a magnificent world of color, beauty, and life. There are different kinds of fish all swimming together in brightly colored reefs. There's harmony. There's awe and wonder. And yes, there's danger and death and disaster, but sometimes you've got to take the good with the bad.
In the end, Marlin has to learn to accept both sides of the ocean in order to help his son. Yes, it's a fish-eat-fish world out there, but the ocean is also a magical place where love and friendship and kindness can be found in the most unlikely of places.
Sure, the world is a scary place, but it's also breathtakingly beautiful.