Depending on the size of grommets used, surface swimming is permitted. I do recommend however that a cap be worn but no plugs are necessary.
The opening of the commonly used grommet is only just over a millimetre and it is unlikely for the water to be propelled through into the middle ear.
Immersing the head under the bath water is not advisable as the soapy water lowers the surface tension and allows water through the grommet more easily. Naturally diving, deep swimming and surfing should not be advised should grommets be in place.
Ear Infections & Ear Tubes