Al was mentioning this in one of his updates years ago about when Splash Mountain was going through its litigious period. I think it was either he or one of the other contributors that mentioned it's actually MORE dangerous to have something like OTSR than no restraints on a ride that involves water. Now obviously there's no way the log is going to capsize or otherwise trap anyone underneath, but it is more psychologically scary to be restrained into a log while it's surrounded by water.
Its physics people...years of calculations and design by engineers and imagineers enable the rider to stay within the log during the drop! Actually being seated would be a good idea if u dont want to get thrown out of the log. I could be wrong tho.