He was wearing his best uniform- the white labelled blue coat, white waistcoat, breeches and stockings of a lieutenant in the Royal Navy, (Uniforms of the Royal Navy, 1795-1812: Uniforms worn by the Royal Navy, from the early stages of the Napoleonic Wars through the War of 1812. The top row displays the full dress uniforms. Bottom row displays the working rig uniforms.)
Replica of a British Royal Navy Lieutenant's Uniform, circa 1800-1811. This image shows a Lieutenant's full dress uniform including a detail of the button twist on the lapels and collar along with the belt plate and an undress fore-and-aft bicorn (chapeau bras). While the 1795 dress regulations established this uniform, its cut matches that of the post-1800 period like at the Battle of Trafalgar.