Sorry to hear of your loss.
Having done a few courses at Mountbatten over the years I drove past it a year or two ago, having "interesting" memories of the place! It was unrecognizable (maybe apart from the jetty), I couldn't even mentally position the Mess.
There is however a quite large memorial on the site by a grassed area and fairly adjacent to the water should you wish it; would probably be a quite appropriate spot and certainly feasible I would have thought - maybe a call to Plymouth council, the undertakers would probably, as suggested above, do that on your behalf?