There (shouldn't) be any blast from the gun firing released inside the tank itself. So, you've got vibration that'll pass through the water. It might increase that, but that's not direct exposure to the blast.
There's still air around the muzzle. Maybe water makes up some of the distance, but there is still a buffer of compressable air in there.
I'm not sure that it wouldn't cause problems, but I'm not sure that it wouldn't, either. I don't think that it'd just be the "depth charge near people in the water" issue.
EDIT: Though some artillery has that recoil-compensating mechanism where the breach moves back when the gun fires. I dunno whether that's true of the main gun on an MBT, but if so, that might cause issues.