The final step in the hydration of an alkyne under acidic conditions is the tautomerization of an enol intermediate to give the corresponding ketone. The mechanism involves a protonation followed by deprotonation.
Consider the following tautomerization reaction:
For the mechanism step below, draw curved arrows to show electron reorganization.
Where relevant, a pi bond has two hot spots: when you draw a curved arrow, use the hot spot nearest the carbon to which the new bond is formed.