The Collaborative International Dictionary of English v.0.48:

Capsize \Cap*size"\, v. t. & i. [imp. & p. p. Capsized; p. pr.
   & vb. n. Capsizing.] [Cf. Sp. cabecear to nod, pitch,
   capuzar, chapuzar, to sink (a vessel) by the head; both fr.
   L. caput head.]
   To upset or overturn, as a vessel or other body.
   [1913 Webster]

         But what if carrying sail capsize the boat? --Byron.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Capsize \Cap"size`\, n.
   An upset or overturn.
   [1913 Webster]

WordNet (r) 3.0 (2006):

capsize
    v 1: overturn accidentally; "Don't rock the boat or it will
         capsize!" [syn: capsize, turtle, turn turtle]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "capsize":
   be lost, careen, come a cropper, culbuter, fall, fall down,
   fall flat, fall headlong, fall over, fall prostrate, flounder,
   founder, get a cropper, go down, invert, keel, keel over, list,
   lurch, overset, overthrow, overturn, pitch, pitchpole, scuttle,
   sink, somersault, sprawl, spread-eagle, stagger, stumble, subvert,
   take a fall, take a flop, take a header, take a pratfall,
   take a spill, tilt, tip over, topple, topple down, topple over,
   topsy-turvify, topsy-turvy, totter, trip, tumble,
   turn a somersault, turn over, turn topsy-turvy, turn turtle,
   turn upside down, upset, upset the boat, upturn