Please Bren, flying boats not seaplanes !
My parents often took us to Southsea during the '50s, and sometimes we would take the boat trip up Southampton Water. I recall that during our visit in the summer of '56, there was an 'American' flying boat moored unusually near the 'Royal' docks ie on the east side, but prior to this, I recall several visits and seeing several 'boats moored on the west side presumably at Hythe. Last time I visited the area was by road in about '87, when the old BOAC hangar and slipway at Hythe had a sign up saying something about an american military unit being based there. I don't recall a 'tower, but then I wasn't looking for one!
Of course, during every seaborne visit, the Princess 'boats were visible at Calshot.