Most of Louisa Reef is underwater; some portions dry at low tide, and some small rocks remain above water even at high tide. Louisa Reef is 128 km south-southwest of Swallow Reef. It rises as deeply undercut walls and steep slopes from very deep water with wonderful stony and soft corals. Safe anchorage is difficult to find, depending on the wind and currents there are sandy areas at the northwest and southwest ends but one must be on constant watch for changes in the wind, the nearest real shelter is the lagoon at Swallow Reef.
Louisa Reef is oval in shape, approximately 1.5 km east to west and 0.5 km north to south. The navigation beacon at its southwest point and the highest visible rock is at the southeast end. The reef itself abounds with pristine coral growth, with a prolific amount of staghorn coral on the north and west walls. The sandy patches south and west of the beacon give excellent night diving.