This behavior is known as "busking" and it is representing a very ticked-off swan. You were in his area, mainly a nesting habitat probably cygnets (baby swans) and he was protecting them by chasing you and your jet ski.

Also, more than likely, when this behavior occurs with a boat or jet ski and you do not readily see the rest of the family, it is indicative that someone has been taunting him and/or his family. Once taunted, swans like any animal, will go after anything that looks like the initial taunting device or individual.

So, if you have children/teens especially, in the area that jet-ski and may run the vehicle into the swan's habitat, then he is intercepting any vehicle to keep them away.

And, yes, you need to be careful in the area so that he does not get too close and get hurt. The Regal Swan