The fabric you were looking doesn't say if it is already heat set or not.
Polyester/ Dacron you want has to be unheat set if you want to get the wrinkles out after stitching.
Most cloth for sale is already heat set because you would be angry it it shrunk after you make something like clothes.

The stuff you were looking at was $16/ yd. The sea pup needs about 3.5 yard which would be $56.
Dyson would be $2.50/ ft (recent price) for 3.5 yard (10.5 feet) or $26.50 + $40 shipping = 66.50.

I personally think $10 extra to know you have something that works is a bargin. It is possible that enough cloth to do two boats would still ship for $40. That works for me in the US.

At that price it would be cheaper (per boat) if you had a reason to get two boats worth or knew someone else who wanted some (never works for me).

I didn't see if the other cloth was wide enough make the sea pup?

Good luck.