Kayak Polo at Thetis Lake - 1974

Here's another blast from the past, but this time with a personal twist. A few nights ago my sister and I were going through my father's old slides. Suddenly, this one popped up.

What the heck? My father didn't kayak. What's going on here?
A look at my father's notes reveals that he took a couple of pictures of a bunch of guys playing kayak polo in nearby Thetis Lake in 1974. If you look carefully in the middle of the picture below, you can see one of the kayakers striking an orange ball in the air with his paddle. You can also see that all the paddles have holes in them to catch and throw the ball, unlike modern kayak polo where the ball is thrown with the hand.

Of course, kayakers use Thetis Lake all the time as it's a great place to practice or to have a quiet and leisurely protected paddle,and there have been organized kayak polo teams in the city in the past, but I've never heard of kayak polo being played at the lake.
So, anyone out there know what the story is here? Was this a one-off gathering? Was Thetis Lake once home to marauding bands of kayak polo-ists? And, perhaps most importantly, is polo-ist even a proper word?