I've not had another swim yet so I've not tested this yet but it suggests you can unlock easily enough. I'm using a Fenix 3 and the details are here - http://endurohub.co.za/fact-13-locking-your-fenix-3s-buttons-its-a-wise-option/
And for a 920 they're here - http://navworld.co.za/3-hidden-features-on-the-garmin-920xt/
I just had a quick search on Google to get the info. Will find out if it works later on this week. What you say about the side buttons makes sense - I certainly sometimes catch the side button when I'm cycling.