Ch'i, if I see you in one of these situations I'll call you "superman" and run for more help.
While confidence is good it could be dangerous to assume our experiences playing with dogs, even those posturing for dominance, have much similarity to their survival or trained attack modes. Get any impolite 60-70lb dog ramped up good while playing and the potential is inspiring. After my dobie stupid I realized I got lucky with a collar, offbalance, and takedown and he probably wasn't doing more than making noise. I really can't imagine a chance against an angry 120lb Rott or adult German Shepherd. They have too many advantages.
So how much does anti-shark armor cost?
This hypothetical stuff reminds me of those, uh, interesting "Shark vs. Grizzly" animal simulations of a few years ago. All physics without consideration of frame of mind.