Three options I can think of, none of which quite fit the bill:

There's a strip of beachfront restaurants on Siloso Beach in Sentosa, but I wouldn't call any of them particularly romantic -- pizza, calamari rings, pitchers of beer, loud music from the clubs, etc.

The Cliff at the Sentosa Resort (ex Beaufort) is romantic and well regarded, but (true to its name) on a "cliff" (well, hill) above the beach, not right on it, so you get views of lit-up container ships twinkling at night. It's also quite expensive.

East Coast Park also has a dozen-odd restaurants by the beach, but they're all across the walkway from the sand, most are downmarket (hawker centre, McD's, etc) and none of them are uber-romantic. Mango Tree does pretty good Indian though in a fairly nice ambience.