I don't mind him complaining about the water bottle - sure, riders throw their bidons away all the time, but across a crowded péloton is probably a bad idea, and besides Cav admitted about San Remo that it was just a terrible day for him.
Complaining about Michael Schär leaving a gap on a descent simply will not do, however, when confronted with the hard and simple fact that a rider who was in the front group to contest the finish had been behind him at the start of the descent (Rojas, as pointed out above). Therefore, at some point between cresting the Kemmelberg and Michael Schär "leaving a gap", Cavendish either sank back in the group (which would be naïve as it makes you more vulnerable to a split like that, and as we know, this kind of thing happens quite often in the Classics) OR José Joaquín Rojas spotted some kind of danger that he thought would split the bunch, and moved forwards in the nick of time (which would suggest that Cav needs to be more attentive otherwise he'd have spotted the same warning signs Rojas had).