I had this situation come up where I wanted to remove a water breathing item and move out of the water in the same month. I didn't know what would happen, so I created a test game and it was fine: I removed the ring in the ocean's watery lab, and moved to a friendly province, and did not die.
But in the actual game, I removed the ring and snuck into an
enemy province. And I
did die from drowning. This was a while back, so it may have been patched.
http://forum.shrapnelgames.com/showthread.php?t=37515