In the 19th century, Caernarfon was the busiest slate-exporting port in the world; quarries in the Nantlle valley and at Llanberis shipped their slate down to the harbour and from there to roofs across the British Empire. The decline of slate ended that, and the basin north of the castle has been turned into a marina, with a slipway, yacht moorings, and a fringe of restaurants and bars.
Several operators run boat trips from the harbour out into the Menai Strait — a one-hour cruise around the castle and across to Anglesey is the standard offering. Larger boats run longer trips towards Caernarfon Bar and the Llŷn coast.