I'm no expert in electronics, but I was wondering about this same thing and researched it a bit.
One problem I thought of was oxidation (rust in the case of iron). Apparently Zinc is one of the slowest oxidizing metals out there and it also conducts electricity. If you solder Zinc to the copper wire and only expose the Zinc to the water it should help it oxidize slower.
I was also interested in measuring the level of the liquid pretty closely. I don't want to just know if it's full/empty or not, but have maybe 5 different levels. I was thinking of using the mechanism of the float switch, but with a longer pipe that goes all the way up. Then inside the pipe put a bunch of resistors and reed/hall switches. Then depending on how much current is flowing through the circuit we can tell how many resistors are in play and what the fluid level is.
A reed switch like this could work:http://www.newark.com/hamlin-electronics/mdcg-4/reed-switch/dp/50F7985