Pete, why I believe the world is a ball.
On the balcony at lunchtime checking the surf through the binoculars, this is what I saw. (been waiting a while for this to happen)

As we ate lunch, the boat moved closer to us, you can see in the pic above the front of the super deck.
about 5 mins later this was our view.
Now part of the bow is visible and more of the superstructure.
another 20min and we had this.

Now the superstructure is side on, about as close as it's going to get, but even more of what's up front and down the back.
This can only be explained by a curved ocean!!!!!!!!!!!!!!!!
No amount of magnification will change how much of this boat can be seen!