zoomkat:
"Smoke alarms are reliable devices from my experience."Wire the level switch across the smoke alarm test button and you have both fire and flood alarm.
Hehehe...yes...can't ask for much more than that now. 
Since I would need to start from zero I'll need a complete starting kit, a (good) welder, LED, wires, etc... The sensor itself is 50€ (I would prefer to use a real one adapted for used water) and I'll need a box to put the whole system when done, button, 220V wire, 220V to 5 or 6V adapter, etc...
Separately these are not expensive, but realistically this is gonna cost much more than 50€ to make the whole thing from zero to finish.
I have the tools to make some concrete, to cut a tree, to disassemble (and rebuild) completly my motorbike, but nearly nothing in electronic ^^