Dude! Literally ANYWHERE! You don't need to go deep either! We could see a lot of fish in Kaanapali just about chest deep water.
There are tours that take you to Molokini, which is fantastic!
Other local spots are Olowalu Beach - right across from the fruit stand. You can park right next to the beach, and again, you don't need to swim too far here either.
Black Rock Beach is right next to all the hotels, and we've seen turtles there in very shallow water along the rocks.