The tourist information office in Viterbo recommended a visit to Lake Bolsena, a short distance north of Viterbo, but it turned out very difficult to reach by public transport.
We opted instead for a rail journey south to Lake Bracciano, the town, after which the lake is named or vice versa, has a castle where Tom Cruise got married:
we spent a pleasant time drinking coffee here, looking in a lovely boutique and down to the shore of the lake where we swam with swans and ducks:
Had a meal practically in the water
Watched a boat race
And admired the view
You don't have to be by the sea to enjoy swimming, Italy has some great lakes.