1 (a) That's wrong. Volenti non fit injuria would not assist B in this case. Participation in a sport involves assumption of the risks normally associated with that sport, but not the risk of avoidable negligence.
The example in the question is based on Rootes v Shelton (1967) 116 CLR 383. The plaintiff was performing a difficult manoeuvre when he crashed into a stationary boat he had not been warned about. The defendant argued that the plaintiff had assumed the inherent risks of waterskiing. The court held that while waterskiing was dangerous, the plaintiff had not assumed the risk of colliding with an obstruction he was not told about.