SORRY! hehe.. been a little hectic with my Maya machine dead

To render particles in software with Motion Blur, you must either turn on the particle's Cache Data attribute or select Solvers > Create Particle Disk Cache. Otherwise, the particle may behave erratically, when rendered.

BUT, ironicly the docs go on to state that:
"General motion blur limitations:
Motion blur does not work with software particles. "

WTF???? ...gotta LOVE Maya sometimes....

Sorry man.

Looks like this is a job for a post-render blur.

So, basicly, you might just have to just render out your particles in another layer, and composite it in post with motion blur using your compositor.


Anyhoo, also I managed to find a tip you might want to think about:


* Usualy, the Inside edges of blobby surfaces seen through other blobby surfaces are not anti-aliased.
Workaround:
Increase Render Quality > particleSamples to 4 or 8 when very high quality is desired.

So, If your water shader is transparent at all, you will want to Increase the particle samples in Render Globals to get a better look.
BUT, since they fly past your camera so fast, I don't think that's too much of a problem here.