Seriously (no offense meant, but seriously) any thief that can't get to Armand without being seen by a guard is a pretty poor thief.
Yeah, he's right. To get to Armand without being seen, put your blue marker on the map where he usually stands, then approach from a south-east angle, swimming across the bay.
S'krivva is harder, because she's in the city, so try Armand first.
If, as you say, you have a clear bounty and are still being attacked, you may have a bug of some sorts. Make sure you have no theiving mods etc installed, otherwise, find your save file, copy it to a safe place on your PC and reinstall Oblivion. Usually works for bugs.