“If it were me, I’d cut him some slack.”
I somewhat agree with this sentiment. As the person who would have to retrieve your dead body, he was within his rights to warn you off.
On the other hand, it was an over-reaction to kick you out because he had trouble getting your attention.
You skipped over the part about the “unfathomable language.” I suspect it was more than simply ignoring the whistle.
Although, as I’ve said, we occasionally see surfers and swimmers ticketed by the police at our beach if they don’t respond to the whistle and come in when told to. We do have a restricted area (military) nearby, which must be avoided.
I'm guessing the "unfathomable language" had something to do with his getting kicked out.