What tennswede said is good advice. I'd only add that if you're nymphing then in order to get the fly deep enough, quick enough it is a good idea to cast the fly above the waterfall and let the waterfall get the fly deep. This is the only was to get a fly down deep really quickly that i know of. Some bigger fish hold at the bottom of those pools right where the waterfall empties into the head of the pool
i guess you could load it up with 5 pounds of lead weight but then you'd just scare all of the fish.