First of all, welcome to Bay12 and to DF!
I laughed a lot with your Minotaur quip.
1. Some dwarves retain relevant skills from worldgen, but NPC fortresses and Mountainhomes are currently waterproof. It could be that they had to ford a river to get to your fortress, but it's something I haven't heard about that often.
In any case, you'll need that novice swimming flair for #3.
2. Grates are useful for many things, usually for sewage systems connected to dwarven showers, because horizontal grates allow fluids and contaminants to go through while dwarves remain in place. You can also create divisions between big rooms without using walls and improve their value, or you can also create a fortification that can be lowered down with a lever, neat enough for marksdwarf shooting ranges. If you're not against checking a game's wiki (and you'll certainly need to check it while playing DF) then read
this piece about grates to have a broader idea about them.
3. There are plenty of things that can be done underground. You can carve a palace out of stone or create a working aqueduct with pumps, grates and channels. If you have artifact furniture you can set up some pieces in key places so that valuable building destroyers are stopped by them (Forgotten beasts, Giant Cave Spiders, something else...) and remain like sitting ducks. If you find magma below, then you can start a serious metallurgy project and start creating steel products en masse.
Godspeed! Keep telling us about your dwarven (mis)adventures.