Ocean flow is known to generate magnetic fields which reach land and satellite observatories with detectable amplitudes. But because the expected signal-to-noise ratio is poor, one must typically know in advance of the data analyses much about the expected behavior in order to identify these small oceanic contributions. We show that in the case of ocean flow associated with tsunamis, a very simple formula (relating both amplitudes and phase) allows the expected magnetic fields to be directly related to the sea surface displacement. Using sea surface data from Jason 1, the formula gives maximum magnetic amplitudes of about 2 nT for the recent Dec. 26, 2004 tsunami which occurred near the magnetic equator. But near the magnetic poles, the maximum amplitudes reach about 20 nT per meter sea surface displacement. Determining the feasibility for extraction of tsunami generated magnetic signals is of interest in designing early-warning systems and for remotely tracking tsunami development.