Caterina greeted us at the Palanca vaporetto (water bus) stop at the arranged time and walked us to the apartment which is a short 5-minute walk. She answered all of our questions about the apartment during check-in and stayed in contact with us during our stay.
The Guidecca island is nice and quiet and is accessible only thru vaporetto from the main island. There are restaurants and 2 mini-markets for grocery shopping nearby.
To/from the Santa Lucia train station, you want to use the Palanca vaporetto stop. To/from San Marco, you probably should use the Redentore stop. Both Lines 2 and 4.1 serve these stops. A single ride costs Euro 7.5 and if you plan on doing a lot of going back and forth and hopping on and off everywhere, a multi-day pass is much more economical. For instance 72-hour pass (clock starts when you validate the pass) costs Euro 40. They also have 24, 48-hour and weekly passes. The vaporetto runs frequently and around the clock (although I think you need to use the N line during certain hours late at night and early morning). The Venice main island is well connected by small foot bridges everywhere and is therefore walkable (at least for me) - it is often faster to walk between vaporetto stops. So you could save some money by just walking in Venice but you still need to use the vaporetto to travel between the main island and Giudecca (if that is where you're staying). I believe the ticket cost for certain point-to-point vaporetto ride is Euro 5.0 including the Zattere-Palanca ride (Zattere is directly across the canal from Palanca). Many places are served by multiple lines and many of the stops (if not all) use separate floating platforms for the vaporetto. Many stops even use separate floating platforms for each direction (going left or right) but the platforms are adjacent to each other. The stops and direction are posted on the platform so make sure you check before you board the vaporetto.