I found something interesting. I was researching Charles Causley's poem Armistice Day. There are a lot of slang words from the Royal Navy in it including the phrase "pneumonia suit". I found the following site which answered the question.

pneumonia suit You'll have to search the page.

The site has a lot of information about the navy that Tom Lewis and Cyril Tawney knew. Check it out.

Here is the poem:
Armistice Day
By Charles Causley
I stood with three comrades in Parliament Street
November her freights of grey fire unloading
No sound from the pale city upon the pale air
Above us the sea-bell eleven unloading.

Down by the bands and the burning memorial
Beats all the brass in a royal array
But at our end we are not so sartorial:
Out of (as usual) the rig of the day.

Starry is wearing a split pusser's flannel
Rubbed, as he is, by the regular tide;
Oxo the ducks that he ditched in the Channel
In June, 1940 (when he was inside).

Kitty recalls his abandon-ship station,
Running below at the Old Man's salute
And (with a deck-watch) going down for duration
Wearing his oppo's pneumonia suit.

Comrades, for you the black captain of carracks
Writes in Whitehall his appalling decisions,
But as was often the case in the Barracks
Several ratings are not at Divisions.

Into the eyes the stiff sea-horses stare,
Over my head sweeps the sun like a swan.
As I stand alone in Parliament Square
A cold bugle calls, and the city moves on.