Yes, I definitely use it indoors in auto multisport mode. Here’s a set of tips, see #13 in particular on practicing with auto multisport:
19 things to know about your Garmin 910xt
Two more tips. I always wait until I’m indoors, then turn it on, or at least when I turn it on make sure I tell it to not use the GPS. Also, you want to go directly to the swim (individual sport) setting to make sure that everything is OK with respect to “indoor swim”, “25 yard”, and distance alert if you are using it (I always set mine to buzz every hundred yards; part way through the length that will be 100 yards at the end it will buzz).
Also, you won’t get bicycle statistics other than heart rate. Even if you get an ANT+ bike and sync up with it beforehand, when you leave for the pool, it will use it’s sync.
Also also, if you want stats from the treadmill, you need the footpad accessory.