From my experience the first two people get really wet. Everybody else gets a little wet but like others have mentioned, if you ride with a couple of large folks, you are likely to get a bit more wet than usual.
This is what we also experienced. The most splashy part was always the drop in the dark, the water comes from the right side and swaps into the log so that the first 2-3 people will have soaked shoes and shorts.
But as others stated above it is always a matter of how the weight of the riders is distributed in the log...
Can Disney adjust the water levels, maybe for the winter so that no one will get soaked?
would be interesting to know if there is a cold-day-mode