The passenger ferry between Sliema and Valletta is both the most practical and the most pleasurable way to connect the two. The crossing takes about ten minutes, costs just a couple of euros, and offers views of the Grand Harbour and Marsamxett Harbour that you simply cannot get from land: Valletta's fortifications rising sheer from the water on one side, the Sliema seafront on the other.
Ferries run from early morning to late evening (roughly midnight in summer, reduced in winter) and depart from the Sliema Ferries terminal on The Strand. The Valletta side docks at the bottom of Merchant Street, just inside the city gate. On a hot afternoon, the ten-minute sea crossing is also the most refreshing route between the two.