We dont have tubes so Im no expert, but my understanding is, yes that is something that you always do while they have tubes. But this is from the mayo clinic website:
If your child has ear tubes ? tiny cylinders placed through the eardrum to drain fluid and allow air into the middle ear ? ask his or her doctor about ear protection for swimming. Some doctors recommend that children who have ear tubes wear earplugs while swimming to prevent bacteria from entering the middle ear. However, routine use of earplugs may only be needed when children dive or swim in untreated water, such as lakes and rivers.