Chill. everyone is a noob at some point!
Main isolator. Do you need it? Personally I'd say yes. I also know boats without.
Possible scenarios: ignition switch or solenoid jam "on"... Battery cable gets chaffed through years of use & suddenly shorts... You eventually succumb to electrical toys, and need to "kill the boat" for storage in order not to flatten the battery... Servicing / mid channel repair... You get the idea.
The biggest failing of these switches is usually water ingress. Keep it dry & it should last gfor a reasonable time. Or buy a f***ing expensive one & it will survive longer.
Yes, you could just disconnect the battery, but you can guarantee that you'll need to do it in a lumpy sea, dropping your spanner which wil lget washed down the deck having just filled the console with salt water as another wave breaks over the bow.....
Pessimist? Nah, jsut an advovcate of a law written by Mr Sod!
Would a "high level" look at electrics be useful?