To my scientific mind, the most obvious difference between whales and fish is that whales are mammals and fish are . . . I dunno, fish, I guess. [Look, I majored in physics instead of biology, so I don't exactly know what the proper taxonomical classification of fish is, but a physics-style comparison between fermions and bosons seems a little over the top for this analogy. Plus a particle based metaphor would make even less sense than the aquatic analogy I am trying to use. But I digress.]
Anyhow, whales are mammals and fish are swimmy things that aren't mammals. Whales breathe air and give birth to live young and lactate, but fish have gills and lay eggs and certainly don't nurse their baby fishes. To this hillbilly living in the Kansas City area with sharply limited oceanic experience, that's the first difference between whales and fish that springs to mind.
If you are wanting to eat something you pull out of the ocean, though, the differences between whales and fish may look a little different.
Moral qualms aside, if you manage to land a whale you can eat from it for a long, long time, especially if you have a frosty place to keep the meat. On the other hand, while catching a fish involves fewer moral qualms for most people, most individual fish you catch will amount to little more than a single lovely meal. Even a fish legitimately large enough to justify a fish story will run out after a few days--especially if you're trying to feed a family.
As an aspiring writer, I would LOVE to start catching some more fish. I've caught a small fish I'm quite proud of, and I've cast several lines hoping for nibbles, but I'm certainly not making fish for dinner tonight. Maybe someday journal submissions and self publishing and maybe even the traditional publishing route will provide a steady stream of fish for my consumption. Until then, I will have to keep my day job.
I confess to the occasional dream of landing a whale, something like Harry Potter or even 50 Shades of Grey, but I can't say I aspire to something like that. That's not something you can plan for, and I personally prefer not to aspire to things I can't plan for. Yeah, it would be great to win a lottery jackpot, but I don't aspire to be a lottery winner; I'm so not planning on that improbable outcome that I've failed to buy a ticket for years in a row. Unlike the lottery, I'm making diligent attempts with my writing--but I'm not planning on even modest success, so I don't bother to even aspire to wild success.
Besides, JK Rowling and EL James didn't exactly head off whaling, yet they found a giant whale on their fishing lines. That's where my analogy breaks down. If you cast a physical line to try to catch a fish, you're not going to land a whale. When it comes to writing, I can't think of a single non-delusional new author who honest-to goodness planned on taking the reading world by storm with that novel . . . even if that novel took off.
As writers, all we can do is cast our lines looking for a few fish. Catching enough fish for dinner is pretty good, even if we never catch a whale.