Maybe strength and watertightness.
5mm is pretty thin. I guess a layer of canvas would strengthen it a bit, but the frame will do far more. A second sheet of ply on the other side of the frame (making a torsion box) would strengthen it even more.
Water protection will come from the paint, not the canvas. What the canvas will give you is a more durable finish than just paint, and it will look great.
Kergan wrote:I have no idea about trimming the edges, I was planning to just use more glue.
The biggest problem with canvas is it needs to be trimmed, because at the edges you just have ragged canvas.

Teak is nice for trim.

And you need to trim around any openings, like the door, and if you can't find a piece of canvas big enough to cover the whole side, you either need to sew a very neat seam (to hide it) or put a piece of trim over the seam, or just live with the seam.
So, not easy, but if you do it right, it will look amazing and give the wood long lasting protection -- on one side.
If you use a plywood that isn't rated for continuous exposure, and moisture gets into it, there's still a chance you could end up with problems. I haven't done any searches on this forum yet, but I bet there are cases of plywood delaminating or bubbling even with an unbroken finish on the outside. That's because ordinary plywood is usually made from trees with very low rot resistance. You get a small leak, water gets into the ply, and it starts to rot from the inside out.
That's why I'm thinking of using marine ply and either a good UV varnish, or oil-based paint, myself. Marine ply is a bit more expensive, but made from rot-resistant species (pretty much all ply is made with waterproof glue these days.) Just seal the edges with epoxy, and touch up the paint when it needs it, and it will last a very long time. There are lots of boats out there with painted plywood
bottoms -- i.e.
below the waterline -- so the combo is plenty good enough for a teardrop.
If I were going to invest all the time and work to finish the side with canvas, I would definitely want to be sure of the plywood underneath.