I think this is the clip.
regarding your question.
This will be incredibly unpopular, but I hope people will read it charitably. Even if you disagree with hitting a horse, his use of negative reinforcement is at least consistent with behaviour change.
Let me state that I'm not a fan of hitting horses to make them go forward, because once you start hitting them, you can't stop until the desired behaviour (i.e., going forward) happens due to the negative reinforcement (hitting them).
In other words, if you make the decision to hit a horse, you can't stop until you get the behaviour change needed. He was consistent with the negative reinforcement and it was immediately removed once the behaviour changed.
I bring this up because I often saw (it's been 10 years since I've been around the show circuit) people hitting horses once in that type of situation, then if it still didn't go, they would stop and go around again (you see this with people trying to load horses on trailers too). In that situation, they are essentially training their horse to do stupider things because a) it won't won't to jump in the water (or go on the float), and b) it won't want to stick around the front of the water(or on the ramp), because it knows that they'll be getting punished for being there, so you'll end up with stranger (and more dangerous behaviour) or won't even be able to get them close to the water jump (or on the float ramp).
I know people will disagree with the purpose of hitting a horse (which is fair - I'm a huge fan of Buch Branaman *), but I thought I'd post that to try to explain why he continued to hit it, as many people would say 'it clearly doesn't want to go in, why is he hitting it'?
By the way, if you'd like to watch a great documentary, which even shows him carrying out actions that some might consider to be 'violence', I highly suggest the following: