Rocks sparking under water
Is this possible? 


Definitely.


  I know you can get sparks from some rocks and minerals - but granite boulders [?]


Yep. Quartz is one of those minerals that "sparks". It is piezoelectric, and triboluminescent.

Here's a wonderful video of this phenomenon: https://commons.wikimedia.org/w/index.php?title=File%3ATribo.ogv

Granite is a rock which has plenty of quartz in it. By definition, granite has more than 20% quartz.


  underwater!!? 


Why not? Those are properties of the quartz itself, not the surroundings. The light does not come from electric arcing of the air, it comes from electronic processes occurring within the crystal. The fact that there's water around it probably does not matter.