Thanks for the replies.
The mortar will be sited about 50 feet from the water and we control everything in the area. As I said in my original post, there's an island right at 420 yards out that I plan on aiming at. However, I don't want to hit it. Instead, I'm going for the humongous splash when the ball hits the water. By my way of thinking, that has got to be much more of a crowd pleaser.
I had originally thought about using 2 oz of cannon grade. Since GGaskill suggested 4 oz but implied that the ball could reach the island, I think I'll start with 3 oz, see what happens and adjust from there.
I appreciate the advice about the wind since that could definitely be a factor. We should be OK though since the wind here in South Florida is generally out of the east and we'll be shooting east to west. I had already done some thinking about the possible effect of wind on the ball and had decided that on the day of the shoot, if it's coming from the wrong direction, we'll call things off.
I absolutely don't have as high an angle on my tube as the one in the picture. That looks like it's almost vertical.
Using a leveling device against the side of the tube I get a reading of 30 degrees from the vertical which I gather translates to 70 degrees from the horizontal. I didn't want to go with 45 degrees since that gives maximum range. I'd like to be able to watch the ball in flight without having it disappear over the horizon.
Looks like I'm going to be spending lots of time bugging the local bowling alleys for junk balls.