Thanks for the post, good to know that's a potential issue! You can also find "one-way valves" made of rubber, often used in for instance the old airhead airboxes, to so water can get out but it doesn't suck in water. Might be a place here for that.

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Grant Johnson
Seek, and ye shall find.
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Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com

It also helps you keep an eye on how much water is still kreeping in.

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Belgium to Nordkapp via Iran | Discover Overland | Destination World

"I have a dream" That one day bmw makes a trouble free final drive!

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Alaska 2014: Hotel Budget ZeroXmas in Death Valley, New Year in Urique

For all you fans who like to ride through deep water here is a fix you might consider .
First though my take on the reason why you get water trapped inside the driveshaft housing . Apparently it does not have an air vent and the rubber seals at the ends are doing a good job - or failing miserably - of sealing .

In either case so much water is getting in because you are driving through deep water with a hot bike . When the water covers the driveshaft it will immediately cool the drive shaft housing and cause the air inside to also cool rapidly . This makes the contracting air inside the housing act like a vacuum pump and suck in water through whatever gaps are available .
It is better to have a good sized vent fixture connected to a plastic tube which has its upper end below the seat , at a level which never gets submerged .
I suspect that the bevel gear housing and the transmission, both with vents , may also be drawing in water at times if submerged . .

The drain hole fix is okay but it too will also ease the rapid intake of water.
Might want to put a rubber plug in it and drain after each water crossing .

Unrelated , but a good example of this pumping effect is with empty steel drums with the caps on but not sealing properly and left outside in the weather while standing upright . The sun shining on the drums will heat pressurize the air and expel it . If a cloud comes across the sun the air cools and the drum begins sucking in replacement air .If it begins to rain the cooling off is greater and the collected rainwater on top of the drum gets drawn inside . Left long enough the drum will eventually fill with water .

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93

I have dream too BMW one day will make R1200GSA with regular swing arm and chain

They do..... Its called a Multistrada Enduro