Howdy Koala.
First - for a temporary repair of this type of situation - just cover it with some good vinyl self adhesive tape - like a duct tape. So you can continue canoeing. My Beth had a repair of this type for a couple of years before I got round to doing anything.
For BIG repairs in plywood boats ... look here 
Now back to small ones
To fix it I would do pretty much as PAR and the others point out. But first I would like to set everyone's brains up for dealing with such an eventuality - it is very likely that every boat that is used will end up with several scars of this type. I love to see a boat with repairs where parts of the skin have been replaced. It gives a feeling of the history. It also gives the feeling that "this is not a dead artifact".
Now ... for the repair.
PAR is spot on - Turn it into a neat hole and make a piece that fits in the hole reasonably neatly.
This particular repair goes down to the chine, so you need to cut the hole so it meets the chine. You also need to use a sander to remove the glass tape for about 25mm (an inch) around the hole.
Be careful with sanding - move the sander a lot to make sure that you watch carefully so you don't go through the plywood outer veneer. Watch what is happening with the glass and the wood.
Make a patch and glue it in like PAR says - but work out how you are going to hold everything in position first - that way if you need holes for screws into the temporary backing plate they can be neat. Do it all once without any glue.
If you precoat the edge using the end grain gluing method in the plan appendices then that will be OK - allow maybe 5 or 10 mins for the precoated resin and hardener to soak in to the edge of the hull and the edge of the patch before gluing. Add glue and now reassemble everything in place.
Wait at least half an hour to before cleaning up any excess epoxy - the ply edge grain is highly absorbant and the excess epoxy provides a reservoir. So clean up at the right time.
Now - the glass tape needs to be replaced inside and outside the hull. However if you can work carefully there is an extra trick.
on the outside of the hull - if you go carefully hand sanding with a block or a sander you can remove some of the outside plywood veneer either side of the join - I would make this slight hollow about 40mm wide and it only needs to be a fraction of a millimetre deep.
Work slow and carefully - to make this slight hollow - it can be filled with glass tape which goes clear and then can be sanded flush with the original hull surface. Generally if you are about to go through the veneer you can start to see some timber grains where the darker glue comes through - you want to stop before this happens. DON'T even try if the outside veneer of the plywood is REALLY thin - you can check from the edges of the cut out section of the hull.
So do this sanding. Then you will have to redo the glass taping of the chine and lay glass in this slight hollow on the outside of the boat.
When that is cured and still green ("green" as in the next day - not sticky at all, but it hasn't reached full strength either) you can use the same method used for fairing down the edges of the glass tape when you built the boat - with the same amount of care to make sure you don't go through the veneer. If you have to hand sand use a block to keep your sanding nice and flat when you are fairing things up.
The glass can make the join fairly structural for canoes and small boats where the ply is fairly thin. With thicker plywood you need to look at a scarfed patch or a permanent backing in plywood or heavy glass (a heavy biaxial or triaxial stitched cloth is ideal for thicker plywoods - the woven glass or tape is really only suitable for repairs in up to 6mm or unstressed areas in 9mm ply). You have to smile at the optimism of people who think that you can buttstrap 12mm (half inch) ply together using a layer of glass tape!!!
Best wishes
Michael
Just compiled some links to useful boat repair articles ... mostly to do with plywood 
How to repair a hole? 
