The wife and I have been at this for a couple of years now, and even won some medals at comps. So we must be doing something right.
What we did is to get one of those little C02 keg chargers, shown here.
The cartridges are $3 apiece and last for quite a few blasts.
We rack to secondary and then shoot a little c02 in there. Works just fine as far as we can tell. Sometimes we even forget to do that. We did our own tasting with score sheets last night and nothing had any oxidation issues.
IMHO topping off with water or anything else for that matter defeats the purpose of making good mead. You measure everything at the begining to get a desired result. Why add other factors later that can make all the difference? Time and aging are also your best friends