External electric field can modify the strength of the spin-orbit interaction between spins of ions in magnetic crystals. This influence leads to a spin wave frequency shift that is linear in both the applied electric field and the wave vector of the spin wave. Here, we explore theoretically the external electric field as a means of control of the spin wave power flow in ultrathin ferromagnets. The spin wave group velocity and focusing pattern is obtained from the slowness (isofrequency) curves by evaluating their curvature at each point of the reciprocal space. We show that the combination of the magneto-dipole interaction and the electric field can result in non-reciprocal unidirectional caustic beams of dipole-exchange spin waves. Our findings open a novel avenue for spin wave manipulation and development of electrically tuneable magnonic devices.