Learn Italian: Anche se tu avessi una borraccia, l'acqua non basterebbe. - Even if you had a canteen, the water would not be enough.

This is the key difference when using anche se. With the subjunctive, it translates to 'even if' and describes a hypothetical situation: you do not actually have a canteen. If we used the indicative hai, it would mean 'even though' and state a fact: you do have a canteen, but the water still is not enough.

How does this sentence fit into the conditional 'if clause' rules?

It acts exactly like a standard hypothetical 'if' clause. The condition introduced by anche se uses the imperfect subjunctive (avessi) because it is an unreal scenario in the present, and the main clause uses the present conditional (basterebbe) to state what would happen in that imagined situation.

Is it necessary to include the subject pronoun tu?

It is not strictly necessary, but it is very helpful here. In the imperfect subjunctive, the first and second person singular forms are identical (che io avessi, che tu avessi). Including the tu prevents any ambiguity and adds a little emphasis.

Is a borraccia just a regular water bottle?

Not exactly. A borraccia is a reusable canteen or flask, usually made of metal or sturdy plastic, like you would take hiking or cycling. If you meant a standard disposable plastic water bottle, you would say bottiglia d'acqua or bottiglietta.