I agree with what bee62 just said , maybe add more than normal open mouth breathing ............ I've never lost one to drowning , but I did find one in the condition you found jaques , it was a C.A. wood turtle , he had apparently gotten caught in an underwater hide , there was a small opening against the side i'd guess he was trying to get to the surface through ........it was a long time ago , he recovered just fine without any problems ........ it did happen similar to what's been said , in that they were put in a new deeper enclosure , I've been real conscious of it since , and been lucky .......... as far as shallow water , it's hard to filter small amounts of water , my two solutions were in ponds or tanks with undergravel filters i'd raise the bottom plates with bricks , or in a pond a dug ledge around a deep spot with a porous cover and gravel .... in tanks without an under gravel filter , a large volume filter , I've got filters that hold up to 7x the water the tank does .......