In April 1793, aged seven or eight years old, Hoste began his naval career as Captain's Servant to Nelson on Agamemnon. Nelson's letters to his wife mention William frequently, calling him "without exception one of the finest boys I ever met with" and "his gallantry never can be exceeded, and each day rivets him stronger to my heart". He was to serve most of his naval career fighting against the French in the Mediterranean and Adriatic. Nelson promoted him to Midshipman in February 1794, and he moved with Nelson to HMS Captain in 1796 where he served with him at the Battle of Cape St Vincent. In 1797, he fought in the Battle of Santa Cruz de Tenerife, where Nelson lost an arm. Hoste was promoted to Lieutenant after the battle. In December 1798, on Nelson's recommendation Hoste took command Mutine, he was still only eighteen years old. It was a post which he held for the next three years, at first under Nelson, and later Lord Keith. Nelson ordered him to Cadiz in September 1805, where he gave him command of the frigate Amphion. However, he was sent on a diplomatic mission to Algiers and missed the Battle of Trafalgar. He only learnt of Nelson's death on his return in November, and wrote in a letter to his father: "Not to have been in it is enough to make one mad, but to have lost such a friend besides is really sufficient to almost overwhelm me". After Nelson's death, Lord Collingwood sent Hoste to command a small force of four frigates in the Adriatic Sea, where he halted the French coastal trade by capturing or sinking over two hundred enemy ships by the end of 1809. On 13th March 1811, his small squadron of frigates were attacked by a Franco-Venetian squadron commanded by Bernard Dubourdieu near the island of Lissa (now called Vis). Dubourdieu's larger squadron of seven frigates and four smaller warships with two hundred and seventy-six guns and nearly two thousand men significantly outnumbered Hoste's four frigates with one hundred and twenty-four guns manned by less than nine hundred men. However, in what became known as the Battle of Lissa, Hoste raised the signal "Remember Nelson" to inspire his sailors, and his superior seamanship and his men's skilled gunnery overcame the larger enemy force. Dubourdieu was killed, one of the French frigates was grounded, and two of the Venetian ships were captured. An island in the entrance to the bay of Vis was named Hoste Island after him. More recently, the Croatian islanders commemorated the game of cricket he organised during the British occupation of the island by founding the Sir William Hoste Cricket Club. Hoste returned to serve in the Adriatic in 1812, and in January 1814, with Montenegran forces, he attacked the French garrisons on the mountain fortress of Cattaro, and at Ragusa (now Dubrovnik), which both surrendered. These actions were immortalised by Patrick O'Brian in his series of 'Master and Commander' novels where they are attributed to Captain Jack Aubrey. To this day the Royal Navy still uses Hoste's famous signal "Remember Nelson" to inspire the crew's moral prior to engagement in battle.