You can't activate the first level ability of a Class unless that Class is level 1. Similarly, you can't activate the second level ability of a Class unless that Class is level 2.

Each Class starts with only the first of its three class abilities. As the first level ability resolves, the Class becomes level 2 and gains the second class ability. As the second level ability resolves, the Class becomes level 3 and gains the third class ability.

You don't have to reveal the card if it's a land card. (Maybe you're not in the mood for fish. That's okay.)

Gaining a level won't remove abilities that a Class had at a previous level.

There's no restriction on how many Class permanents you can control, whether they're the same or different classes. Each Class permanent tracks its own level separately.

Each Class has five abilities. The three in the major sections of its text box are class abilities. Class abilities can be static, activated, or triggered abilities. The other two are level abilities, one activated ability to advance the Class to level 2 and another to advance the Class to level 3.

The level 2 and level 3 class abilities of Fisher's Talent will replace any Fish or Shark tokens you create, not just ones created by the abilities of Fisher's Talent.

The token's characteristics are entirely replaced by either 3/3 blue Shark creature token or 8/8 blue Octopus creature token, as appropriate. They don't have any other abilities the tokens would have been created with. Anything else specified in the effect creating the tokens (such as tapped, attacking, "That token gains haste," or "Exile that token at end of combat") still applies.

Gaining a level is a normal activated ability. It uses the stack and can be responded to.