Travel from Venice Airport to Venice city (Island)

You heard wrong. It's a four minute walk from where the bus drops you off to the front of the train station:
https://goo.gl/maps/PpzxrCVSbwp

You might consider getting a good guidebook. The Rick Steves one for Venice is very good.

Happy travels.

If you are landing at 230PM and are coming from outside the Schengen zone, having time to clear immigration and get to the island portion of the city to get on a dinner cruise boat that leaves the dock at 430PM is a bet I would not make.

OK. why don't you just tell us the name of the hotel. Yes, you can probably make it. Most flights into Venice are connections from elsewhere in Europe where you will already have cleared Schengen immigration. Customs is nothing, just walking out the door marked "Nothing to declare." Buses to Piazzale Roma go every 15 minutes, M-Sat, 20 minutes Sundays and holidays. It takes 24 minutes, then a 5 minute walk over the bridge to the train station.

So you land at 2:30, walk out the door with your luggage at 3:00 and take a bus at 3:40 and be at Piazzale Roma at 4:04 and be at the train station at 4:10. So you decide. What you do with luggage? Time to check into your hotel? Maybe store at "Keep Calm Point" next to the station.

You could also get a taxi at the airport to Piazzale Roma. That would save a bit of time, maybe a half hour for about 20 or 25 EUR. A public bus is 8 EUR.

I did the trip twice yesterday due to going back to the airport to meet my daughter. I used the ATVO express bus and it worked great. The actual driving time was 15-20 minutes.
If you are flying from another European country as a connection, you walk right out. I came from Lisbon and it was just about an hour from touchdown to arrival at my apartment in Venice. This included waiting for landlord and walking 10-15 minutes to the apartment.

Two weeks ago, we landed at Marco Polo at 13:30 on Emirates. Took a full hour from touchdown to exiting the terminal. This similar to what we have found on several previous trips.
The ATVO bus leaves at 20 minute intervals, at :00, :20, :40, costs 8 euro, terminates at Piazle Roma. From thereto the station is a five or ten minute walk across the Calatrava bridge.

Where exactly does the dinner cruise leave from? Which hotel are you staying at, I assume you need to drop luggage before cruising.

If you have carryon luggage only, you will get out of the airport much more quickly. Just thought I'd add this small detail in case it gets overlooked.

There is a lot of pertinent information in https://europeforvisitors.com/venice/articles/venice_airport_buses.htm.

I too recommend using the bus and walking to your hotel. If you have a smart phone, download the Google map and use that to help you find the correct way immediately. Or ask for help - as a man myself, I know that can be daunting. If you are female you probably wont have a problem - at least my wife doesn't.

the other side of the island

I guess technically this is true - if we talk about the right island. Venice (the city) is build on 118 islands (https://en.wikipedia.org/wiki/Venice), so it is "on the other side" of one of them, but the distance is about 400 meters :-)