The main issue with repairing plastic hulls is that is that most adhesives don't bond very well to the plastic. For temporary repairs duct tape is the way to go. Its quite adhesive and waterproof. If the hole is too big use the duct tape to secure something else waterproof (e.g plyboard or plastic) in place. If possible try and do both inside and out.
If you want to do a permanent repair there seem to be two options:
Repair in a similar way to fiberglass. I.e. Sand the surrounding area and apply a fiberglass patch and epoxy. The one significant difference is that the area around the hole should be flame treated before applying the patch by lightly heating with a propane torch or similar. This helps the epoxy bond to the surface. Be careful not to burn your boat though.
The alternative, more conventional, way is to melt/weld more plastic into the hole. I suspect this is what a professional would do if you asked them to do the repair. Here's another good article on it. For smallish holes you want to get some polyethylene welding rod (presuming the boat is polyethylene) for large holes you need to get a patch. The basic idea is to melt the rod into the hole using a hot air gun or hot metal. As you've melted two of the same material together this repair should be very strong.
Disclaimer: I'm mainly a dinghy sailor so haven't actually repaired any plastic hulls. This is just based on my general understanding of boat repair and reading the internet.