Can methane cause ocean acidification?

No. methane does not cause ocean acidification directly. It dissolves in water (sparingly) as the CH₄ molecule and does not dissociate (react to form H⁺ and CH₃⁻) or otherwise react with water.

Methane could cause acidification of the oceans *indirectly*, though, if the methane is converted to CO₂ by methanotrophic microorganisms (i.e., bacteria and archea that "eat" methane and respire CO₂ ).

CO₂ behaves quite differently from CH₄ when it dissolves in water. It actually *reacts* with water to form carbonic acid:

CO₂ + H₂O -> H₂CO₃ (H₂CO₃ is carbonic acid)

H₂CO₃ is a weak acid, and dissociates:

H₂CO₃ -> H⁺ + HCO₃⁻

and to a lesser extent:

HCO₃⁻ -> H⁺ + CO₃²⁻

As the concentration of CO₂ in the atmosphere increases, the concentration of carbonic acid (and its dissociation products, which include H⁺) increases. This is a direct consequence of Henry's Law. The acidity of an aqueous solution depends on the concentration of H⁺ ions; as the concentration increases, so does the acidity (the pH decreases).

Why can't you just Google this?