A.C.
Each doctor has a different opinion...and it also depends on the type of tubes they used. Some are a larger diameter and allow water through more easily, others are small enough that there also has to be some pressure to break the surface tension.
I agree that your doctor should have given your instructions on "water precautions". Generally speaking, I would say it isn't necessary for treated water, BUT if he just got them two weeks ago he should be avoiding water in his ears period.
Have fun.