How deep should my water be

I just got a three month old dbt. His shells is prob only about an inch inch and a half. I set him up in a 20 gallon long tank. How deep should the water be for this little guy? I read somewhere turtle can actually drown. I found that odd. But also read elsewhere these guys are great swimmers. I have prob about 5 inches of water in there now, but that doesn’t allow me enough water to use the heater. Please advise what is the best depth of water for a dbt at this age and size.Thank you