I doubt you'd notice much of anything happen if you dumped your drugs into a swamp.
Edit: we don't use swim, swamp, my dog, etc here
Are you asking because you want to take a a dxm containing syrup that also contains phenylephrine? I noticed they sometimes mix the two. Normal therapeutic doses of phenylephrine are like 10mg...
I don't think it's a good idea to take a high dose of phenylephrine and dxm together. If your trying to trip on dxm then you should really just buy a dxm only product.