We were told we didn't have to use ear plugs. My son has had tubes twice now and he has never once had ear plugs in. But, he also never goes underwater. Our doctor said when you shower you don't get enough water in your ear to make a difference, and if you bath only dump cups on water over his head to wet it and rise it, otherwise keep the head out of water. In the summer, pools are safe to go underwater in without ear plugs, anything without chlorine is not.
Tubes tend to fall out in a year. Do you have any check up appointments? Or when you go to the ped. ask them if they are still in. If they are not in, it's safe to go under without ear plugs.