Purging submersible with nitrogen
Using argon is, in effect, a costly physical path to remove oxygen, hence your interest in using nitrogen.
However, I suspect, a more efficient, cost-effective, and likely safe chemical path is also possible. For example, assuming a control structure (so as to mitigate potential deaths from O2 depletion, as has been reported, for example, among sewer workers working in a recently drained iron pipe), oxygen scavenging can result from a limited exposure to iron surface previously scrubbed with an acid. Here, lemon juice appears to be a friendly, pleasant, and effective choice.
Note, iron powder and ferrous salts have been used in the food industry to remove problematic oxygen (see, for example, THE USE OF OXYGEN SCAVENGERS AND ACTIVE PACKAGING TO REDUCE OXYGEN WITHIN INTERNAL PACKAGE ENVIRONMENTS).
One should be able to stoichiometrically compute, and experimentally validate, a safe amount of Fe (or its surface area) to employ. This is to limit a continuing unsafe scavenging action if, for example, subsequent maintenance work is required.
You can also employ commercially available products containing Calcium Chloride, for example, to remove water vapor.