First open-sea lighthouse

The first offshore (aka sea-washed) lighthouse was the original Eddystone Lighthouse built in 1696–98 (first lit on 14 November 1698) approximately 18 km (12 mi) off Plymouth, Devon, UK, on a shallow reef known as the Eddystone Rocks. The structure was wooden, octagonal, stood 80 ft (24 m) tall and was lit by 60 candles. The project was overseen by British merchant and engineer Henry Winstanley, who had lost two of his ships on these perilous rocks. Winstanley had to rebuild his lighthouse, changing it to a dodecagonal shape and slightly increasing its height in 1699 after it suffered weather and wave damage. This reworked lighthouse was washed away altogether (tragically along with Winstanley and his team) in the Great Storm of 26 November 1703.

Three more iterations of the Eddystone Lighthouse followed Winstanley's: the second, built by John Rudyard also from wood, was completed in 1709 before burning down in 1755. The third lighthouse on this site was designed by civil engineer John Smeaton and was this time constructed from granite between 1756 and 1759. Smeaton's lighthouse was operational until 1882 with the upper section relocated to the nearby city of Plymouth to be added to a lighthouse-shaped monument (Smeaton's Tower) in tribute to the celebrated engineer. The final iteration, which remains to this day, was completed in 1881, overseen by William Tregarthen Douglass.

The oldest still-active lighthouse at sea is Bell Rock Lighthouse, some 18 km (11 mi) off the east coast of Scotland, UK, built by British civil engineer Robert Stevenson and completed in 1811. The lighthouse is built from granite and sandstone and measures 35.3 m (116 ft) from its foundation to the top of its lightroom. Bell Rock Lighthouse has been operational since 1 February 1811 and was automated in 1988.

Depending on your definition of "offshore", another contender for this title is Cordouan Lighthouse in the Gironde Estuary of western France, which was built in 1611 (and extensively restored in the 18th century), making it the oldest active lighthouse in France. While situated some 7 km (4.3 mi) off the coast, it sits on a small island in the tidal zone where at low tide large sand banks (and subsequently a manmade causeway) are exposed providing access, so technically it is not permanently surrounded by the sea like Eddystone or Bell Rock.