Definition of "dunker"

Active sonar, for aircraft purposes still largely associated with dunking, suffers less variability but often has less range. […] Development of both dunkers and sonobuoys continues towards lower frequencies, implying bigger arrays, and deeper deployments.

1986, Jane's Defence Weekly, volume 6, page 1285

Then they drop the dunker into the water and spin it around a few times so you end up upside-down.

2013, Philip Kaplan, Naval Air: Celebrating a Century of Naval Flying, page 179