Best throw for a very steep downhill tunnel shot is a putter. You need to experiment to find the one that will hold the line perfectly at the required speed and angle to get to where you want to land. I have had little success with drivers as they flare out and skip a mile. Mids work OK, but it is easier to float a putter in my experience. With the water behind the basket, you really need something that will not get away from you and will not skip/skid a mile after landing (that would be a putter). Try a mildly beat in Aviar.
BTW, just to give you a point of reference, most steep roofs on houses are 45 degree slope (12/12). It is almost impossible stay on those roofs doing the crab walk with your butt, hands and feet down.
This. If it is longer than you can get your putter (this would have to be exceedingly long if it a steep hill) then a mid. Just make sure you throw down the hill, not straight out.
EDIT: Crab walking on roofs sucks, but when I tried not to I immediately dropped to my hands as well before sliding uncontrolled down the shingles...killed my palms