Take it from a 17 year upholsterer...don't use canvas duck. You don't want anything that's waterPROOF. You want water RESISTANT. If it's water proof, the temperature change will allow moisture to accumulate and there will be mold & possible rot before you know it. Not to mention the odor when the cover is removed. Water resistant allows air to enter & leave while eliminating the moisture from edew & heavy rains/snow.
Sunbrella is pricy, but there is an equivilent. It's called Dickson Marine fabric. It's available with or without the plastic backing (get it without). Any upholstery shop can order if for you for a small upcharge. try eBay of iOffer.com for leftovers. You boat is going to take 15-20 yards of material, depending on your design and any mistakes (I call them adjustments

).
When creating your cover, put a couple of vents in it. That is simply a "clamshell" shape with screening under it that allows the air to flow better without allowing water in. You will also need access to a heavy duty sewing machine. Most home units won't cut it as they aren't equipped with a walking foot. UVR/Nylon thread is also essential.
Get online and get ideas from other pictures. It really helps when deciding where to start. the first cut is the hardest.
Best of luck!