Basic approach - large high res ground plane. Plug in a Wave2d node into displacment with u/v pos set to one corner and then as wimvdb says animate the phase node or use the frame number in the variables number to change phase (not sure units seem quite small). One node by itself won't look too good so thought is to add together several wave2d nodes with varying origin, amplitude and frequence to create more interesting effect (one wave2d node to be the main wave and the others to cut up the surface)

Beware using displacement maps will slow down the render quite a bit and add a water material won't improve things.

Seem to remember this has been covered before so worth a forum search if not already done.