According to Bronsted-Lowery concept of acid and base, Acid is that substance which donated H⁺, while, Base is that substance which accepts H⁺.
Hydroiodic Acid being a strong Acid with Ka value of 3.2 × 10⁹ is when dissolved in water it donates H⁺ to water and water acts as a Base. i.e.
HI + H₂O → H₃O⁺ + I⁻
H₃O⁺ formed is the conjugate Acid of H₂O, while I⁻ is the Conjugate Base of HI.