Santa Croce is one of the 6 areas of Venice so it encompasses a large area. You will need to get a specific location for picking up your keys. Rio Tera San Leonardo is located in the Cannaregio area, which includes the train station and is walking distance from Piazzale Roma. Piazzale Roma is the end of the line for land vehicles and is where taxi’s and buses deposit visitors coming from the airport.
The best way to get around in Venice is to walk, but when that is not practical such as when arriving with luggage you can take the Vaporetto. The Vaporetto is the public water transportation system in Venice. There are multiple lines that service most of Venice with predetermined stops. You can buy a single-use ticket or a multiday pass depending on your anticipated usage.
The closest Vaporetto terminal to your hotel is the San Marcuola Casino stop. With 2 elderly travelers, this may be your best bet, although you will still have about a 5-minute walk from the terminal to your hotel. You should contact them to get specific directions from the Vaporetto terminal.