There shouldn't be any confusion as they serve different purposes.
A whistle is just another cue, like your voice or a hand signal. A clicker is used as a distinct and precise marker that dog can easily build associations with specific behaviors. Your voice using a particular word can also be a marker, but is far less precise as a clicker as variation in tone, pitch, speed can make it a little more difficult for a dog to build associations. For my young cocker I do use clicker training and also employ a specific word, "yes," for verbal marker training.
I enjoy clicker training as there are a variety of ways to teach and reinforce behaviors - shaping, luring, capturing, targeting. Dogs that are clicker trained tend to pick up new behaviors far quicker than traditional marker training with your voice and it's amazing the great variety of behaviors and the level of precision you can teach a dog with a clicker.
However, I never used clicker training for any gun dog training. Having a whistle, a dummy, a check cord, and a rambunctious pup was already enough of a handful that I would never consider bringing a clicker along or any sort of treats. I don't usually recommend clickers for training for the field unless the dog isn't a natural retriever in which case you should backchain the retrieve, but many people eventually run into problems with spitting dummies out when a dog is too eager to get the treat. Honestly, if your dog has good drive, having a clicker or using treats are things that really aren't necessary if you're just shaping behaviors that already come naturally to them.