Boats break. All sorts of plastic eventually breaks. If you learn to weld with a heat gun, you can keep a boat going for a long time.
If you think the broken boat is a manufacturer's defect, then BEFORE YOU DO ANYTHING, take photos of the damage, note the serial number, contact your local dealer and ask the dealer to help you make a warranty claim.
If you were a super lozer and pulled a huge aerial mcgoon that broke your boat on a rock, after you landed from 20 feet up (or if the boat is older) then you're on your own. Gather your tools. It's time to weld.
You'll need a heat gun with various nozzle accessories that will concentrate and shape the heat for better control over your welds. You will also need a flathead screwdriver, a burnisher (if you don't have one of those then an old spoon will work too), some rough sand paper, and the most important material: HDPE plastic cut into very thin strips (the thinner the better)
Drill the corners of the break so that it will not continue to run.
Sand inside the break. This will increase the chances of a good weld. It makes small burrs to melt into the welding material, and if the chemical bond doesn't form, then you'll have more surface area for a mechanical bond.
Be careful if you need to pry the crack open a little bit. The plastic will bend a little bit, but you don't want to push it so much that the crack wants to get bigger.
Heat the inside of the crack, only to the point where you see the white colored plastic (stressed) start to turn clear or back to the color of the rest of the boat.
Give the entire area a couple of passes with the heat gun so it is warm; still warm enough to touch, but too hot for a sensitive part of skin like the inside of your wrist. This prepares the boat's plastic to melt with the repair stick.
Preheat one of the thin pieces of HDPE until it gets glossy and mostly transparent. Only uncolored HDPE will get clear. Colored plastic will be ready when it gets glossy and very flexible, almost runny.
Keep working the repair material into the crack, alternately heating with a heat gun, and blending the two plastics together with a burnisher (or spoon). Don't try to mix too much. This is plastic, not liquid. Just a little manipulation to make the weld stick.
DO NOT let the boat get so hot that the plastic starts to run, or you'll get a concave dent in the boat. It must be hot enough so that it can accept the repair plastic. Too cold and the repair will just come apart. Too hot and the boat is ruined. Have patience. If you try to heat it too fast, you'll burn the boat. It will discolor and start to get brown. Patience.
When the structural repair is done, I like to use ultra thin pieces of plastic to cover an area beyond the damage. This acts like an extra patch for back up. Some people go with a frankenstein's monster stitch look, as if stretching a bunch of butterfly bandages across the crack. That will work too. I prefer the all-over patch because I can eventually add more plastic layers and reshape anything that was distorted.
Once everything has cooled off (give it at least an hour), I like to use a surform shaver to knock down all the odd high points, and make sure that the corners of the repair are sticking to the boat well. Hit the edges of the repair to see if they will peel up. If they do, then go back in with the heat gun and make them stick down.
Once everything is sticking together nicely, be sure to shave the corners of the patch down so they feather into the boat's plastic well. This will prevent any rocks from catching that edge and concentrating any extra force on the repaired area.
Hit the whole thing with a heat gun to quickly melt down any small burrs. You CAN NOT melt it enough to make it all smooth without melting the boat. So, just clean it up a bit and quit. It's a repair with a heat gun. It won't look perfect. If you need perfection, go to a professional plastic welder with your boat and get an estimate.
Welding is, in principle, simple. But, no matter what material, it is also an art. The only way to get good is to practice. Replace your boat every few years. Plastic gets brittle with time. But, don't throw your old boat away. It will be good welding practice.