The hyfrofoils leave from downtown Buenos Aires regularly (south of the main train station - it's not a pleasant walk across several major roads with manic B.A. drivers), taking about 45 minutes each way. They're pretty swank, with duty-free shopping (which is nevertheless very expensive), televisions, and food. Slower, cheaper ferries take about two hours. Once you get to Colonia, it's about a 1/4 mile walk to the historic district. The newer section of town is pleasant enough. If you have time, you might want to consider going by bus from Colonia to Montevideo, then taking a hydrofoil from Montevideo back to Buenos Aires.