I made the mistake of putting this "Dophindasher drives great on water" theory to the test on Shy Guy Beach, online. It cost me dearly. I haven't driven any waters elsewhere with it yet, but so far the theory isn't holding any water for me.
As to the poll....hmm, wait one second...
* AurorX goes off to check the stats of the Mach Bike.
I prefer speed.

Too bad it doesn't work with you. But yeah, the mach bike really is a speed bike. What I hate though is the amount of time you lose getting bumped. You've countered that technicality with looking back but you sometimes can't anticipate the attacks of others that much.
Speaking of the dolphin dasher...it's not really faster on water. But doing water shortcuts actually make it more likely to win. Just don't overdo it.