Marta wrote:Thank you for a thorough explanation

The Black Water reservoir/lake that I have in mind, however, is a made-up place.
What I'm looking for is just a straightforward tr*nsl*t**n of that name to Gaelic, given that Black is a surname - not the colour (I'm not sure if it makes a difference?).
Whether it's a real place or not is irrelevant. If you want a good tr*nsl*t**n, you have to base it on real usage, or you get something which isn't real Gaelic, and if you don't want an accurate tr*nsl*t**n, why would you even bother asking for one?
Uisge MhicGilleDhuidh, as suggested by Akerbeltz, would be a bad tr*nsl*t**n, because it would mean "Black's River" to any speaker of the language. A more appropriate tr*nsl*t**n would be
Loch MhicGilleDhuibh.
Seonaidh wrote:I don't think Kielder Water can be said to be in the Lake District - and Rutland Water certainly isn't. So "uisge" would probably be OK in Gaelic.
OK, I was wrong about that bit, but they're still lakes in England with English names, and they say nothing about Gaelic.
To date, I have seen "uisge" only in the names of several streams/rivers, and never lakes. If anyone knows a counter-example, I would love to hear it.