PlantFiles: Bigleaf Hydrangea, Lacecap
Hydrangea macrophylla 'Sea Foam'

From the sources "Hydrangeas for American Gardens," by Michael A. Dirr (2004), and "Encyclopedia of Hydrangeas" C.J. and D.M. Van Gelderen (2004 - Timber Press):

Older variety with Gray-green leaves that grows from 5' to 6' tall. Flowerheads are sparsely covered with ray-flowers, usually white to light blue or light pink. This cultivar is tender and needs protection in less suitable places, although it does well in seaside plantings. Supposedly close to wild subspecies H. macrophylla var. normalis, which was named by Wilson at the time of the introduction. Origin Japanese.

Flowers run from white in alkaline to mauve-violet in acid soil. No zone hardiness noted. AKA 'Maritima'