Hey,
I'm working on a water leakage alarm project and I need a water-presence sensor that can wirelessly transmit data to an arduino (via Bluetooth or wifi or another medium). I couldn't find any 
Any suggestions?
is the water that is being monitored outside? in a bucket? soil moisture?
do you want to know the exact level or just if its low?
the most common responce to this question is "GIVE MORE DETAILS"
trust me, i've been asked that many times
Google "wireless water presence sensor" for lots of examples.
I've seen where old smoke alarms have been easily converted to water leakage alarms.
Saves on wireless part too. 
Saou:
I'm working on a water leakage alarm project and I need a water-presence sensor that can wirelessly transmit data to an arduino (via Bluetooth or wifi or another medium). I couldn't find any
That can only mean you didn't really search. Or you don't know what you want. Both are possible as details are lacking.
How much water do you want to detect? (when there are a few drops at a specific location or when it's getting knee deep or anything in between).
What wireless option do you want to use? (distance and environment are key to deciding this).
Does it have to be battery powered or do you have mains power available?