A quick search reveals this topic has been covered before here with some valuable insights provided so I'm just adding my findings to what's been said already.
Bike is V7 111 Special for reference.
Pulled the swingarm as I knew there were possible issues with water getting inside the casing and there was a total of maybe 50ml in there mostly pooled down the bevelbox end and heavy condensation over everything else - drive shaft, uni, inside of swingarm casting. Some surface rust on the pinion retaining nut and a minor bit on the uni joint.
IMHO and as smithswede and others have mentioned the problem relates to the way the rubber bellows is located into the swingarm. A sort of internal circlip inside the boot pushes the boot into a corresponding groove in the swingarm casting. The clip is not a full circle however, there's a gap of about 25mm between the two ends when installed so not only is 25mm of the boot is not pushed into its groove the 25 mm portion of the boot not covered by the spring is actively pulled away from the groove leaving a gap that you could easily slide a match stick through.
It doesn't help that this is in about the 11 o'clock position either. As smithswede did I've sealed it all with silicone and put a small vent hole on the lowest portion of the boot. Seems to be zero problem with the attachment of the boot to the transmission case despite the fact it's got a cheap and nasty cable tie securing it. I couldn't see any evidence of water entering on the flange where the bevel box mounts.
For the record the driveshaft splines were dry on the uni joint end and lightly greased on the bevel box end.