Page two of the instruction manual gives the answer.
"In order to directly connect a Watermark sensor to a
datalogger, the sensor must be equipped with an in-line blocking capacitor (CSI
model 257 sensor, $90, see below)."
Sounds like it's a basic probe requiring AC drive.
This has been covered before. A search should show up quite a bit.
(or wiat for eclectic to find it for you
)