Ah so you are going to fit the PVC into the Hobie rod holder to support the shade. Great Idea, post some photos when you are done.
The top of the rod holder hole on my Adventure Island is 45 or 46mm (sorry Im in metric) I think that converts to 1 3/4" at the top of the hole and as Sherminator said, it narrows slightly to the bottom.
The Hobie rod holder extension inserts are 42 or 43mm at the bottom of the taper and 46 mm at the top so they jam in fairly firm.
So 1 1/5" outer diameter PVC would just fit, but do they measure water pipe from Internal dia? Take your tape measure with you to the shop.