No,no,no. Go to a local farm store and get the white electric fence tape. IT has multiple stainless wires woven into the plastic threads. Would be perfect for your project.
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It sounds more logical than aluminum, but the fact that stainless steel doesn't corrode is because when it oxidizes it creates a passive film of chromium etc. oxides that protects it.
Have you seen stainless steel in electronics?
Just go with gold plated material if you don't want to try to reinvent a wheel.
Just for exposed part, rest of your wiring can be regular copper cable.
Thanks for the suggestion, but they seem unreasonably cheap, $10 for 200 meters? Surely there can't be that much metal in there.. I will most definitely look into those locally!
I assumed it wasn't used because of its poor conductivity as compared to copper or aluminum. Gold plating isn't really an option because my "sensor", or I should call it system, is expected to be able to cover the full length of a water supply line, which varies but is likely in the range of tens of meters - meaning I need to be able to detect a leak spanning anywhere across the entire length. Even if I expose say every 10cm of wire that would still add up and also introduce complexity to the system, since I'd need to get some sort of PCB to get that gold plating and you can see how this becomes a problem. Does stainless steel become an insulator once it oxidizes?
Here's what I found:
The presence of this oxide layer can slightly reduce the conductivity of the stainless steel compared to its unoxidized state, but it does not entirely prevent electrical conduction. Stainless steel remains a good conductor of electricity overall, so while oxidation may change its electrical characteristics to some extent, it will still conduct electricity. However, the conductivity can vary depending on the specific alloy composition and the extent of oxidation.
Dot like aluminum, but yes, that layer always reduce conductivity. Depends on composition of ss how much. You can test it. Neither copper becomes "non conductive", actually it conducts quite well even if oxidized.
Graphite?
In the chemistry lab when we're doing electrochemistry and want to avoid any corrosion products we either use platinum or graphite.
Platinum would probably be a slightly better choice than gold and it costs less than half.
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There are various conductive threads made for wearable circuits including stainless steel, silver plated thread (has a conductive oxide), carbon, etc.
More than enough to stop the deer and the neighbor's horses! I think it is at least 5 strands. Some of mine have been up for 15 years and most of the PVC strands are gone, but the SS wire still is conductive. There are a thousand different formulas for stainless steel.
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When you find my name in there smile!
I suggested gold as it is easier to acquire then platinum at least in my area and plating it is easy.
Here is a source on graphite wire: Graphite Wire Supplier | Stanford Advanced Materials I have never used them but it looks like a good starting point.
The tape sounds like it is worth a try and per @Paul_KD7HB you should get several years out of it.
But if this is the case, then what am I worried about in the first place?
Because it's still getting significantly less conductive. But not like aluminum that makes "ceramic" kind of oxide coating that doesn't conduct.
Easy to experiment with multimeter and piece of oxidized copper...
There are 2 different things being discussed as if they are the same thing:
How conductive different metals are; copper is much better than stainless steel.
How well the interface between various metals and water with ions in it conducts, which depends on the ions in the water and the chemical behaviour of the metal.
I think @Delta_G knows far more about this than I do.
Because eventually it will corrode all the way through and the connection will be broken.
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Silver plated wire might work, tin might be as good, cheaper and more available.
If you plumping is copper, you can use a single wire taped right to the pipe.
I strongly suggest you perform a small experiment with some wire and a container of cement. I doubt it will perform as you hope. I am NOT a cement expert but cement in contact with the ground is likely not perfectly dry. This is why basements in most parts of the country feel "damp".
You could also only power the wire for a millisecond every 5 seconds or so. this will further reduce any current induced damage to the wire.
Look, If you know exactly how the "sensor" is going to be placed in the wall, it is easier to give some suggestions. The difference of being poured in concrete or placed in brickwork mortar is like day and night. Both, for leak detection and oxidation of the sensor.
For my particular site this time it will be mortar. Matter of fact most likely it will always be some kind of mortar..
Hi,
If you have an air gap, then the danger will be the gap filling with oxidised copper/silver whatever and causing a low resistance across the open gap.
So a sensor that uses current/resistance is not a good idea.
A capacitive would be better.
Tom.. 
PS. How cheap is cheap? How important is the reading and over what time span?