Many years ago I had a few games of Canoe Polo, so I guess I will give some feedback based on that.
The playing area is too large, canoe polo only uses 35m x 25m approx., with 5 players a side. Our boards are really that much larger so having a field over 3 times the size would make for a very slow game... fast games are exciting.
I would suggest a water polo ball, reasonably light, float and easy to hold in one hand.
I'm not sure how you paddle and hold the ball if not between your legs, might work better to be netball style, no paddling with the ball and pass with-in 5 seconds. The skill then is in the off the ball play rather than on, getting into position for a pass, leading etc.
The only other thing I would include is mandatory PFD and full face helmet, both will offer protection against being hit by a paddle. No matter how friendly the intent is, it only takes one rush for a ball for someone to get hit hard. I imagine a paddle to the head would not only hurt but could be fatal.
Could be fun, but after seeing how trashed a Kayak gets from polo I'll take an inflatable board please