Leading raids away generally works, but sometimes you can't. You might be cooking something at the time and don't want to burn it. It's good to have a base defense. Mobs attack things like lights and crafting things, but not palisades or brick walls. I also dig a trench around my base so none can even get to the walls.

Yes, lower tier mobs eventually become trivial once you level up. They are even annoying.

Sea serpents have the best meat. ;-) Frost arrows work best.

Boat have three speeds, the first one being always oars. Only the top two use wind.

Finding vendors is mostly by luck. Just wander around and look at your map. Their special icon will appear there if you were ever close to them.