I recently visited the Turtle sanctuary in Malacca, Malaysia and an Malayan Box Tortoise was in ! metre deep water.Quite happily walking around on the bottom and sitting under a tree trunk. I asked the same question and was told because there heart rate is very slow,about 15 beats per minute,they can take a breath and stay under water for a while.The Tortoises we have in our back yard stay under water for at least 10 minutes. best wishes Peter
