Ako mi se sutra bude pila samo voda, ponijet ću bocu sa sobom i preskočiti kavu.

Bude pila is future II (also called the future exact).

In Croatian, when you have:

• a future condition introduced by ako = if
• and a future result in the main clause

the subordinate clause often uses future II, while the main clause uses future I.

So here:

• Ako mi se sutra bude pila samo voda = If tomorrow I feel like drinking only water
• ponijet ću bocu... = I’ll take a bottle...

The pattern is:

• ako + future II
• main clause + future I

Here, bude is the auxiliary, and pila is the past participle form that goes with it.

A simpler comparison:

• Ako bude padala kiša, ostat ću doma.
  If it rains / if it is raining, I’ll stay home.

So bude pila is not random—it is the normal tense choice in this kind of future conditional sentence.