I always considered Vanguard
to be the most hard sci-fi Trek that Pocket has done so far. Especially the first 3 books.
Anything by Christopher Bennett is a good answer too though.
I was surprised that Over a Torrent Sea
got fairly negative reviews on this board (rare for your books). I rather liked it. For some reason, it reminded me of a David Brin novel I had read not too long before it - Startide Rising
. Not that they had the same plot or anything, just that they had some interesting similarities - a hard sci-fi book with a starship exploring a water world, where the protagonist starship had a mix of human and aquatic crewmembers. (Dolphins in this case.) This book felt similar in a way that's hard to explain.
I haven't read Watching the Clock