You are always welcome to book tours from outside vendors whether
DCL's are filled or not. Some people find it is less expensive to book on their own and not use DCL's tours but that may depend on the size of your group and the activity. If booking on your own, make sure you know what their cancellation policies are in case you don't make it to port for any reason. Also what happens if they can't get you back to the ship in time (which in Alaska isn't usually a problem on land based touring, but helicopter and float planes may be a different story). If there is a specific tour with a specific vendor you are looking at, post a thread and people who have used that vendor may be able to give you more specific feedback on the good/bad of the tour their took.
You know Viator is just a booking site. They contract with local vendors to provide the tours similar to what DCL does (and that may be fine for you, let them pick the vendor). For Alaska, it might be just as easy to skip the middle man. Many here would give you recommendations on the tour companies themselves if you let us know what you are looking for. For example if you are looking for whale watching in Juneau, Viator won't disclose the vendor until you book, but many here have used Harv and Marv's and had excellent experiences.