"The waves, they're very pretty to look at but very dangerous," said Barry Manuel of the Halifax Emergency Management Office.
By early Sunday, the storm had maximum sustained winds near 85 mph (140 kph) and was moving 26 mph in a north-northeast direction.
The Obamas delayed their planned Sunday morning departure from Andrews Air Force Base to Sunday mid-afternoon because of the weather, White House aides said. The worst of Bill was expected to pass east of Martha's Vineyard before the Obamas arrival.
As plans changed Saturday for the first family, nearly all south-facing beaches on the island were closed to swimmers and large signs blocked roadways to shorefronts. Lifeguards used caution tape to rope off access points, and police patrolled the beach to enforce the closings.
"The concern we have now is that the riptides are very strong," said lifeguard James Costantini. "There's a very strong undertow."
The same high waves that worried safety officials, however, had surfers buzzing.