All of Avro's aircraft leaked badly. On 205 Sqn in Singapore, where crossing the ITCZ meant flying through very heavy rain, considerable amounts of water would enter the aircraft, and a local mod involved drilling holes in the floor to avoid excesive build up.
On ships, water shifting around inside the hull causes instability, usually laterally.(think Herald of Free Enterprise) Also google free surface effect.
On aircraft, the fuselage being long and relatively narrow, the worst effects are in the fore and aft. Those big bits you had to climb over in the Shackleton fuselage, and which they refer to as the main and rear spars, are actually nothing more than baffles to stop the water from rushing back and forth and making the aircraft uncontrollable in pitch.
It is not generally known that Avro built a prototype flying boat to compete for the Specification R.2/33 (which gave rise to the Sunderland). It was built in great secrecy at Woodford, taken in bits to a secret hangar next to the Manchester Ship Canal, and assembled.
Came launch day. It slid down the slipway into the water, and just went on going down, leaving nothing but a few bubbles. The whole thing was hushed up,and Avro never built another flying boat.