The prospect of landing a Sunderland on ice recalls an earlier Belfast-built craft. It was called the Titanic ... Seriously, fresh water avoids salt corrosion problems although the boats were all thoroughly protected with yellow jointing compound oozing from all faying surfaces. (I noticed some when visiting the IWM Sunderland many years ago, and the Belfast freighter was built the same way.)
Routine maintenance was done in the open, the majors in the hangars, more to protect the aircraft innards than the grease-monkeys. The Catalina base at Killadeas had a canopy at the end of the slipway just big enough to cover a pair of Twin Wasps.
As to maintenance on the buoy, my father told me that officialdom questioned Pembroke Dock's remarkable consumption of linen rigging cord for metal aircraft, and was told that the fitters tied every tool to their wrists. Otherwise, as Chugalug says, it was lost forever.
For a Sunderland-on-land-landing, see
www.youtube.com/watch?v=cD00dqRbKzc