(FORTUNE Magazine) – William E. Simon is spending some enchanted evenings in the South Pacific. Since mid-February he has been sailing the waters around Fiji, Tahiti, and the Cook Islands in his 125-foot ketch, the Freedom. But this is no ordinary romp through the tropics. Simon, 61, is recuperating from open-heart surgery done in January. With the help of an artificial valve in his chest, the former Treasury Secretary and energy czar is energetically scuba-diving and island- hopping with his wife and some guests. ''When I get back home,'' he says, ''I'll be like a tiger.'' Just like always. Simon engineered the first big LBOs of the early 1980s and made some very large profits. More recently he has been buying thrifts in California and Hawaii. But his cardiac crisis was not brought on by his fast- paced lifestyle. He had rheumatic fever as a child. The disease left him with a defective aortic valve that went unnoticed until he flunked a stress test. Simon, who receives highlights from the Wall Street Journal and the New York Times on his yacht's fax machine, plans to return to the U.S. in late April. Aside from a long vacation, he says there was another beneficial effect from his surgery: ''It proved that I had a heart, which surprised a lot of people.''