When you convert your fluid to a polygon mesh, Maya generates velocity per-vertex data that can be used to compute motion blur when your fluid is rendered using mental ray for Maya renderer. Velocity Per Vertex, a fluidShape node attribute, generates data from the fluid object by mapping fluid velocity values to R, G, and B color values. Velocity Per Vertex is on by default

The velocity per-vertex data is passed to the output mesh through the polySurfaceShape node’s color set named Motion Vector Color Set. By default, this color set uses the velocityPV data generated from the fluid object.

To see motion blur in your rendered fluid, ensure that you have Motion Blur on in your mental ray render options. See Motion Blur in the Rendering guide.