Here in Germany there is a company they doing it. They work with a system called Low pressure wetsandblasting(translated)
They work with 3 bar pressure and sand with 0.09 to 0.25 milimeter for antifouling they also prepare the hull for osmosis works. They sell also the stuff you need I post the link from there website. But it is in German but maybe you can call the guy and ask him. http://www.sandstrahl-gang.de./
Good luck

Here in Germany there is a company they doing it. They work with a system called Low pressure wetsandblasting(translated)
They work with 3 bar pressure and sand with 0.09 to 0.25 milimeter for antifouling, they also prepare the hull for osmosis works. They sell also the stuff you need. I post the link from their website. But it is in German but maybe you can call the guy and ask him. http://www.sandstrahl-gang.de./
Good luck

As far as "walnut shell" blasting, I have had it done before & my 42 Post is getting it this week.
It is less harmful that sand blasting but there will still be work to do after it is blasted.
It opens up and small voids in the gelcoat which will need to be filled & faired out (useless if your trying to strip the paint & buff out your gelcoat).
People often have it done when their boats develop small blisters.
This opens up the blisters & they let the bottom dry out. Then they repair where the blisters were & roll on a barrier coat so water can't penetrate it again.

Except for walnut blasting same experience as DJD..
Have done two boat bottoms with sandblasting and I would think that if you're already in the business you have a good feel for the system.One practice on an old clunker and you can come and do my next one !