Stockholm is a city built on water, so it makes sense that ferries are a perfect way to travel, and you will find that they are equally popular between local people and tourists. You can use an SL Card, or you can buy single tickets on the quay.
Depending on where you want to travel depends on which ferry you will take. The M/S Emilie will take you to Djurgarden, and then to Hammarby Sjöstard, which is the most modern district in Stockholm. This ferry runs daily throughout the high season and at the weekends during spring and autumn, and is wheelchair friendly.
The Djurgarden White Ferries are a regular sight in Stockholm as they transport people to Skansen, the Rosendal Gardens or Waldemarsudde. Throughout the spring and summer, they also transport people to the Gröna Lund amusement park, as well as to Skeppsholmen.
The Nybro ferry will take you from Nybroviken to Vasamuseet, Gröna Lund, Skeppsholmen and Stadsgardskajen. You can purchase return or single tickets that are valid for 24 hours.