This from the extremely helpful National Martime Museum:
"In 1874 Mr Wilde's 'electric light' was tried in HMS Comet, a corvette (a type of warship). An improved 'electric light' was fitted in the battleship Minotaur in 1875. In the next few years a 'searchlight' as it came to be called was fitted in all new ships, the first being HMS Alexandra completed early in 1877."
[Source: 'The Electron and Seapower' by Vice Admiral Sir Arthur Hezlett, 1975]