Just above the falls, on the west bank. Probably near your hotel is a quiet area great to enter. You enter in shallow water, but it drops to about 80' very fast. I've also entered below the falls and swam upstream when there was LOTS of flow. The river is wide there and you don't feel any flow. Well as soon as you approach the narrows it's like a solid wall of water. You can't move any further North. Really kinda cool.
The visibility of the river can be anywhere. Just depends on the flow of the river, time of year, runoff and so forth. That's why I wanted to try it in winter, cold but quite clear. Just the fish gave me strange looks like "why the heck are you here, we have to be."