Can Grove Water sensor be interfaced with raspberry pi even without the Grovepi/Grovepi++ module??
Strictly speaking the signal on that sensor is analog (changes as a function of water level), however it is claimed to work with digital pins, indicating water presence. It will probably work in the same way with RPi GPIO pins directly. It's hard to predict what the threshold level will be, i.e. how much water is required before the digital pin toggles to "water present" state.
If you need to measure water level (as opposed to simply detecting there's water), you'll need to convert analog signal from the sensor to digital. You can certainly use something else than Grovepi to do the conversion, but this is a rather broad topic.