The high-speed train from Venice to Milan will take about 2.5 hours, which means you will arrive at Milan Central Station at about 11:30 am. If you immediately took a train from Milan Central to Como (about 35 minutes) you might be able to get there by 12:30 pm to 1 pm depending on when the next train to Como leaves. If you want to be at the airport by 16:30 for your flight this does not leave much time to see anything and then get to the airport. The ferry from Como to Bellagio with the Hydrofoil will take 45 minutes, but you would need a round-trip ride so right there you are spending 1.5 hours on the ferry. Personally, I do not think you have sufficient time to do this. I checked to see if there are any direct trains from Como to the Bergamo Airport and it does not look like it. All of the trains from Como to Bergamo involve at least 1 transfer and take upwards of 2 hours.