DasTactic, Attica, and Perun are all good youtubers for learning the game.
I find Das's videos to be particularly newbie friendly. He has a whole series explaining the entire game, with this one being a good starting point. His series covers just about every aspect of the game, including how to get yourself underwater as a non-UW nation. I'd scan his playlist to see particular topics that interest you.
As a general rule, you want to basically eat up as many independent provinces as you can grab, especially high-income provinces. A good number to aim for is having about 12-ish provinces (including your capital) by turns 12-15. Hitting this number involves practicing with a nation and figuring out how to make the most use of their vanilla troops to effectively take down independent provinces.
There's no official diplomacy system, but depending on what personality the AI is given, they usually won't attack you if you respect your borders with them while expanding into indie provinces. Once you or them attack one another, it becomes an all-out total war to the death in most cases.
For your getting wet predicament, if you can find any mercenaries that happen to be amphibious, that would be ideal. Otherwise you can often recruit amphibious independent troops from certain coastal provinces, like mermaid soldiers or turtle men. There are forgeable items that can get you underwater, but in the early expansion phase when you only have access to items that give personal water breathing or breathing for like 5 guys, it can be useful to just give one to a mage or super-combatant who can take down a province defense by himself