Samwolf is correct, and it's pretty silly. It's bad enough having to tell my crew to go to bed - now I have to dress them too!
However, depending on how you have your deck gun modded, your crew might not ever put on raincoats. The file that controls under what weather conditions the deck gun can be used also controls when raincoats go on.
In Ubisoft/SilentHunterIII/data/submarine/NSS_UboatXX/NSS_UboatXX.cfg you'll find the line
StormConditions=7,0.1max wind speed [m/s], max rain intensity [0,1]
The 7 sets max wind speed after which you can't use the deckgun and the raincoats have to go on; the 0.1 does the same for rain intensity. Your values might be different, since a number of mods change this line. If you want the raincoats to go on more easily, you'll want to adjust these downward, but be aware that you will then lose use of the deckgun more easily. I personally don't use the deckgun much, and like the appearance of the raincoats, so I'm okay with leaving this value at stock.