Roger Pratt, 62, from Warwickshire, was killed and his wife Margaret injured when armed men boarded their yacht off the town of Vieux Fort on Friday.
A post-mortem examination showed he had been beaten and drowned.
St Lucia Police said it was questioning two more men about his murder. Three others were still being held it said.
Mr Pratt, a retired engineer, and his wife, from Moreton Paddox near Stratford-upon-Avon, had been on a round-the-world voyage since June.
Police were called to reports of a robbery on board their yacht, the Magnetic Attraction, while it was moored off the island’s south coast at about midnight on Friday.