Water leaking through canvas zipper.

wax the zipper

I use a block of Bee's wax to rub regularly on my Camper door zippers and it does work .I couldnt believe when i recieved my new Cub Daintree that the door and side opening Zippers didnt come with a flap over them.Cub told me to erect my awning ! !

I'd be most surprised if any zipper would hold pooled water such as I think would happen as your photo shows would be possible with the bars supporting as shown.
If it is possible, I'd shift the bars which are parallel to the zip so they don't form a pool, or if it's possible, lower the outer edge so there is sufficient pitch to prevent that pooling.

Bob

I think your only solution would be to use 2 extra poles, either upside down or with those plastic caps. When it rains you need to elevate the zip away from the ponding water.
The upside is you only need them when it's wet.