Under normal circumstances, no, leviathans don't land on water. While they hail from a naval construction tradition, being able to put gun mounts and hatches in the lower half of the hull is just too useful. Accordingly, any Lev forced down onto water will sink - quickly in the absence of any lift from its electroid tanks, from water leaking in through every casemate and hatch.
It would be entirely possible to build an "amphibious lev" - no hatches or gun mounts on the lower hull & sides. It's probable someone did it, but then failed to come up with a good reason to make another. Lev captains prefer to land in a suitable landing cradle, but have no problems "anchoring" - low power can keep a Lev safely afloat several fathoms above ground without gobbling too much coal, and (particularly for the bigger ships) it's possible to land on any moderately soft surface, as long as it's dry. Voila - instant 'cradle'.
Assuming our amphibious lev existed, taking off would be a problem - the hull would be wetted, so you'd be lifting against the additional mass of water. But possible. Movement on water though would be very limited - water's much thicker than air, and the ducted fan systems aren't optimised for that work, and control surfaces meant for air might not handle water well.
W.
