Most Vernier sensors are not meant to be completely submerged in water. In general, sensors that measure some parameter of water, e.g., pH, conductivity, or dissolved oxygen, should only be inserted a few centimeters, or as directed in their user manual. They are typically not waterproof beyond the handle or any type of connector. Sensors that were not designed to interact with water, e.g., CO2 gas, force, or voltage, should not come in contact with water at all.

A few exceptions include:
Extra-Long Temperature Probe (TPL-BTA) which can be submerged up to 30 meters.

Soil Moisture Sensor (SMS-BTA) can be submerged up to the BTA connector (approximately 1 meter).

Surface Temperature Sensor (STS-BTA) can be submerged up to the BTA connector (approximately 1 meter).

PAR Sensor (PAR-BTA) can be submerged up to the box containing the electronics (approximately 5 meters).

Vernier Optical DO Probe (ODO-BTA) can be submerged up to the switch box (approximately 1 meter) for up to 30 minutes.

Go Direct^® PAR (Photosynthetically Active Radiation) (GDX-PAR) can be submerged up to the box containing the electronics (approximately 5 meters).

Go Direct^® Optical Dissolved Oxygen Probe (GDX-ODO) can be submerged up to 1 meter for up to 30 minutes. However, note that submerging the sensor impacts both the pressure sensor reading and the Bluetooth radio performance, so is not recommended.

If using wand-style Go Direct sensors wirelessly in the field, the Go Direct® Sensor Clamp (GDX-CLAMP) is a useful accessory that will help prevent accidental drops and submersion.

For the most complete information, check the user manual for the sensor in question. If it includes no reference to being waterproof or water resistant, assume that it is not.

Related:
Are there options to protect my LabQuest from getting wet?
Can I use Vernier water quality sensors in salt water or in a salt water aquarium?
What do I need to do to prepare for outdoor data collection?