The correct form is Freezes because the sentence expresses a scientific fact or universal truth.

১. Scientific facts are always stated in the Present Simple Tense regardless of the main clause.
২. ‘Will freeze’ implies a future prediction, which is unnecessary for a constant natural law.
৩. When the condition (when water freezes) results in a fact, zero conditional rules apply.

অতিরিক্ত তথ্য: Universal truths never change their tense in conditional sentences.