Ok, Coopershooter, I'll bite. The water clarity is the thing I'd consider first. Since you saw the fish when snorkling, and describe the river as spring fed, proper depth should be easily reachable.
More of a problem to get something to them, at the depth they're at, what it is is less important. They'll attack a ham on a ski line, when they get going.
Might I suggest a float rigged to allow the line to slip to a predetermined depth? Easier to find fishes preference if your dangling something in front of their noses. Tough job! Wish I could be there to help! Woodbutcher