The past year has seen numerous publications underlining the importance of a space mission to the ice giants in the upcoming decade. Proposed mission plans involve a similar to 10 yr cruise time to the ice giants. This cruise time can be utilized to search for low-frequency gravitational waves (GWs) by observing the Doppler shill caused by them in the Earth-spacecraft radio link. We calculate the sensitivity of prospective ice giant missions to GWs. Then, adopting a steady-state black hole binary population, we derive a conservative estimate for the detection rate of extreme mass ratio inspirals (EMRIs), supermassive black hole (SMBH), and stellar mass binary black hole (sBBII) mergers. We link the SMBH population to the fraction of quasars f(b)(in) resulting from Galaxy mergers that pair SMBHs to a binary. For a total of 10 40-d observations during the cruise of a single spacecraft, O(f(bin)) similar to 0.5 detections of SMBH mergers are likely, if Allan deviation of Cassini-era noise is improved by similar to 10(2) in the 10(-5 )-( )10(-3) Hz range. For EMRIs the number of detections lies between O(0.1) and O(100). Furthermore, ice giant missions combined with the Laser Interferometer Space Antenna (LISA) would improve the localization by an order of magnitude compared to LISA by itself.