I think soldiers only ever drink the water whenever they are on duty, when they are off duty -which all your soldier dwarfs should be when there is no threat- then they drink beer and train.
Nope. You tell them to carry water, they drink water. And then get sad about it.
An ambush is a small number of enemies (less than ten) that are invisible until spotted, like thieves, but somewhat easier to detect. The alert message is "An ambush! Curse them!" They skulk around the outside of your fortress, looking for a target of opportunity. They will often attack caravans as they move to your depot. Ambushers have random weapons, and typically have a leader (with a career title of "guard") with a different weapon from the rest.
You didn't mention that ambushes will almost always appear while you have a caravan, or that they will TEND to move straight towards your fortress same as a siege. On some maps where going outside was lacking in attraction I've had ambushes detected right at the gate. Ambushes also tend to come in twos and threes (of squads) at a time, although if an ambush party doesn't approach the fort while invisible it may not be detected until much later.
Ambush squads that are detected will apparently always go for your entrance as long as there's a way in.
Traps will catch ambushing goblins but they don't seem to reveal them. What seems to happen is a goblin will get brained by a trap, a dwarf will run out to grab the corpses' socks, he'll run into the rest of the squad and THEN you get the warning. Sometimes there's 6-7 dwarves out there fetching dead gobsocks. Pray the goblins aren't crossbows or bows.
Ambush squad leaders aren't always "Guard". Sometimes they're just "axegoblin", or et cetera. I think the difference is whether you're at war.
If ambushing goblins kill their detector, make sure you forbid the victim's items toot sweet, otherwise you'll get dwarves marching out to steal his boots or die trying.
Because goblins that are visible and determined to assault a fortified gateway guarded by traps, your military, and caravan guards are a lot less of a threat than goblins invisible and lurking outside you might want to NOT shut in your trade depot. Fortify it, yes, but don't close the door/raise the bridge while you have a caravan.