Open water swim on the 910xt uses min/mile in the details pane, and in splits.
It shows in mph in the speed graph, and activity comparison table.
I saw somewhere that some endurance swimmers liked min/mile.
For most user's though they are only using open water swim rarely - either races or specific open water practice, and the bulk of training is done in the lap pool. For these users, seconds/100 meters/yards is far more recognizable - and that is really dependent on your "home" lap pool, not your choice of metric/imperial for other sports.
I'd like for open water swim pace to be based on a user preference setting in GC of time/mile, time/100 yds or time/100 m. If you want to offer MPH go ahead, but that one is REALLY useless. This setting should be reflected in the details pane, the speed graph, the activities list display,dashboard, and splits.