Take A Good, Deep Breath. End of a Gosport Era.

Didn’t John Noakes from Blue Peter do it once?

The dunker was bad enough - screw this!

W.
Folk at British Antarctic Survey had to do the dunker if they were going to use HMS Endurance's helicopter. Plenty of tales about it there. But the main problem people spoke about was the total disorientation, such that it was quite difficult to remember where you were in relation to the exit.
 
I.K. Brunel hit the same problem when building the dry dock in which SS Great Britain was constructed - the dock structure "floated" in the soft ground surrounding it. I forget how they solved the problem, but it's mentioned in the book I have about her.
And several London dock walls collapsed when the docks were de-watered for Mulberry Harbour construction during WW2.
 
The harbours were a major work of civil engineering. The caissons alone used around 31,000 tons of steel and 1.5m yards (1.4m metres) of steel shuttering. They were built in hastily constructed dry docks in the Thames and Clyde rivers.

The caissons and other components of the harbours were pulled across the Channel by tugs and assembled off the French coast. They were operational within 12 days of the landings.

Mulberry harbours
 
Thought most of those were built around the Solent, bit of a drag to get them around from London during wartime innit?
Some came a lot further than that.
Construction took place in the Graving Docks at Middlesbrough, Goole, Southampton, and of the Port of London Authority at Tilbury. Several units were built in an entrance lock at Plymouth, and in the floating dock there. Two London Docks, the East India and South Dock, were de-watered after precast concrete block dams had been constructed in the entrance locks, and 18 units were built on these two sites, where hundreds of thousands of cubic yards of blitz rubble were used to make construction beds on the dock bottoms. Twenty-three units were constructed on slipways built on the foreshore at Stokes Bay, Stone Point and Langston Harbours in the Portsmouth and Southampton areas. Here the caissons were constructed to about 60 per cent of completion and were then towed to jetties where the remainder of the construction was carried out.
Source
 
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