Photo of SVP style drifter:

The SVP drifter is a "holey sock" drifter - the large blue mass (the drogue) will unfurl and sink to 10m depth (about 30 feet). The drogue is attached to the transmitter in the white/red ball at the surface via a wire. The ocean currents at 10m depth push the drogue through the water.

Photo by Larson King.