Our 2 year-old daughter had tubes for the second time last week. The ENT said we only needed plugs if we were planning on going into lake water.
We've not needed to use plugs in the bath or the pool over the summer. She was clear of ear infections from last Feb (2009) until this past January when both of the tubes dislodged and caused fluid to build-up (not from being in the bath).
Depending on how long it's been since the surgery, I'd really suggest calling the ENT to verify. We have been told both times (by the same ENT with many years of experience) that baths, pools and ocean water are OK - lake water is the concern because it's the only one not naturally able to prevent the growth of most bacteria that cause the infections.