Hi,
As rule of thumb, you should set the time step in order to have a Courant number between 1 ad 2.
The Courant number Cr is given by : Cr=Cg(fp)*Dt//Dx
Where Cg(fp)is the group velocity associated to the peak frequency, Dt is the time step and Dx is the size of the smallest element of the spatial mesh.
The group velocity can be calculated from the dispersion relation. In the deep water case, the group velocity is approximately equal to 0.78/fp, while in shallow waters Cg=(g*d)0.5, where g is the gravity acceleration and d is the water depth.
Hope it helps,
Giovanni