retrolefty:
Agreed, 'pure water' is a good insulator so that actual resistance of the water in your basement while not 'pure' would tend to have highly variable resistance and the input led of an opto takes several milliamps of current to conduct and with just using a 5 vdc voltage source I would think that it would be a very unreliable method to depend on. There should be several circuits on the web to show decent circuits of the proper sensitivity for your use.
By pure water you mean distilled water right? I thought the OP was talking about tap water (At least that is what I thought when he talked about water in the basement). If it was about any kind of water (including distilled), my fault.
Edit: I just measured the resistance between two wires underwater (tap) at 10 cm distance between them. The resistance was 7M o.o. I though tap water was much more conductive. I would do what CrossRoads proposed experimenting with different values for pullup resistors.