how do i get from a hotel in san marco to the san basilio terminal in venice
Ask the hotel how to arrange transportation. You will not be the first. You may be able to walk as the cruise terminal is that far from the train station.
Google San Basilio Terminal Venice. Trip Advisor has several posts on this. Check with your cruise line (I presume you are taking a smaller ship). Most will have either a shuttle from the water bus (vaporetto) stop to the ship or some other accommodation. If you don't want to take the water bus you're probably looking at an expensive water taxi. I have seen luggage porters bringing in luggage from a Celebrity cruise to hotels in the San Marco area so the same arrangement (in reverse) may apply to your cruise.