According to the OED:
ground-sea. A heavy sea in which large waves rise and dash upon the coast without apparent cause.
* A. 1642 Sir W. Monson Naval Tracts ii. (1704) 247/2 He met with so great a Storm and Ground Seas.
* 1756 Prince in Phil. Trans. XLIX. 642 A rumbling noise was heard, like that which usually precedes what the sailors call a ground-sea.
* 1835 R. S. Hawker Prose Wks. (1893) 28 On, through the ground-sea, shove!
* 1865 Englishm. Mag. Oct. 296 A heavy ground-sea.
a. ground-swell. A deep swell or heavy rolling of the sea, the result of a distant storm or seismic disturbance.
* 1818 Scott Hrt. Midl. iii, The agitation of the waters, called by sailors the ground-swell.
* 1840 R. H. Dana Bef. Mast i. 2 The vessel..rolled with the heavy ground swell.
* 1877 Black Green Past. xxviii. (1878) 221 Crashing its way through the rolling waves of a heavy ground-swell.
b. fig. Usually with reference to mental or political agitation.
* 1817 Coleridge Zapolya i. Wks. IV. 219 It is the ground-swell of a teeming instinct.
* 1856 R. A. Vaughan Mystics (1860) I. 91 The religious world was rocking still with the groundswell that followed those stormy synods.
* 1870 Lowell Among my Bks. Ser. i. (1873) 219 The deep-raking, ground-swell of passion, as we see it in the sarcasm of Lear.