Parameterization of submarine melting represents a large source of uncertainty in modeling ice sheet response to climate change. Here we present in-situ observations of melt at near-vertical ice faces using a novel instrument platform mounted rigidly to icebergs. We investigate boundary-layer dynamics controlling melt across 31 observational segments that span a range of forcing (1-12 cm/s flows over 3-10ºC). While melt generally scales with velocity and temperature, we find substantially enhanced melt when forcing is unsteady. Several implementations of the 3-equation melt parameterization show melt can be predicted within a factor of 2 if the model is evaluated with peak near-boundary velocities and flows are quasi-steady. However, if flows are unsteady or the model is evaluated with low-resolution velocities, melt is underpredicted by 2-100x. We conclude that understanding the detailed character of near-boundary flows is critical for submarine melt predictions.