Not since the classic song "Cat's In The Cradle" has a story about a daddy-son bonding given us such feels.
While we know that dads can come in all varieties (although they seem to be united by a common love of cheesy jokes) classically, a father tends to represent certain ideals. Dads have authority, power, and strength. A dad provides for his little offspring. He protects them. He gives piggyback rides and kisses bruised knees…but he's also an ever-strong, ever-inspirational source of wisdom and pancake recipes.
But that's an ideal. And here's the thing about ideals—they never really match reality.
Marlin's struggling with the way he failed his wife and children during that freaky barracuda attack. He wasn't there for them. He didn't protect and provide. And because of this, he's hell-bent on not letting anything happen to Nemo…even if it means keeping lil' N under proverbial lock and key.
You know the saying that parents need to give kids roots and wings? Marlin's a-okay with the "roots" part, but he doesn't really want to give Nemo wings. (Or should that be fins?)
Of course, meeting another dad in the deep blue sea makes him rethink all this:
CRUSH: Aw, it's awesome, Jellyman. Little dudes are just eggs, leave 'em on the beach to hatch, then coo-coo-ca-choo, they find their way back to the big 'ol blue.
MARLIN: All by themselves?
MARLIN: But, dude, how do you know when they're ready?
CRUSH: Well, you never really know. But when they'll know, you'll know, you know?
By the end of the movie, Marlin has realized that he'll never be the ideal dad. He can't keep everything bad in the world away from his son. It's just not possible.
But it's also not preferable. Nemo needs to go out and explore the world. He's got to try to make it on his own and swim with his own two fins. The only thing Marlin can do for his son is to love him a whole bunch no matter what. After all, that's what good dads—human or fish—do.