INSTEAD of joining the more popular uniformed youth organisations like the Road Safety Patrol and Boy Scouts, marine-lovers Chan Ho-ki and Joseph Ngai Chi-kwan have opted for something special. They joined the Hongkong Sea Cadet Corps instead and have learned a lot about seamanship, engineering, sea survival and naval customs. Ho-ki, 18, a second-year printing student at Kwun Tong Technical Institute, has been with the Sea Cadet's Anson team for four years. ''It's really exciting and enjoyable; it's different from other usual uniformed groups. The corps provides rifle-shooting and the opportunity to learn and at the same time have fun on ships,'' he said. ''I like the sea very much. When I am aboard I feel free savouring the beauty of nature.'' Joseph, a fifth-former at Lung Cheung Government Secondary Technical School, aims to join the marine police upon graduation. He has been with corps for five years. ''The training on leadership, discipline and duty teaches me how to be a good seaman. The more I pick up knowledge, the more I fall in love with marine life,'' he said. The Hongkong Sea Cadet Corps was set up in 1968; at present it has 12 units with over 700 members of both sexes, aged between 14 and 18. The corps is not a pre-service organisation but it has adopted the methods of drill and discipline used by the Royal Navy. Each of the 12 units is affiliated to one of the ships of the Hongkong Squadron, which enables the cadets to spend time at sea in the ships. The corps has two training centres by the sea, one at St Stephen's Beach in Stanley and the other at Tsam Chuk Wan in Sai Kung. ''Apart from normal scheduled training and outdoor activities, we encourage service to the community like assisting local charitable organisations in fund raising and getting cadets to take part in crowd control service,'' said Mr Leung Sui, executive officer of the corps. Application forms for the corps are available at its headquarters at 11, Fung Shing Street, Diamond Hill, Kowloon. For more information, call 327-7883.