I've had some success repairing several water butts with fibreglass but it's a tricky business. As other contributors have suggested, getting the fibreglass to stick to the plastic is a challenge and, with a full butt containing 200kg of water, there is a lot of pressure to test repairs plus the butt flexes, which tends to loosen a patch. However, it can be done, so if you are interested read on...
I wear disposable gloves and safety specs, as things can get messy with fibreglass. Thoroughly clean around the crack inside and out- I use alcohol to degrease; detergent and water would probably work but it must be completely dry before fibreglassing. Next rough up the surface- I do a couple of quick passes with a wire brush in an angle grinder. Cut a fibreglass sheet patch (using scissors) with plenty of overlap. Make up the resin with hardener, paint on area around the crack, stick on the patch and liberally apply resin on top, making sure it is well soaked and there are no air bubbles. The resin sets in just a few minutes so needs applying quickly and with confidence- maybe a test piece beforehand would be a good idea if you are not familiar with it. I aim to put a patch on both inside and out. Recently I've also painted a flexible roof repair paint on to the outside of the butt, I don't know how much it adds to the repair but it looks good and I have a spare tin in the shed. Leave to dry and wait for the next bit of heavy rain to see how successful it has all been.
Just getting a new butt is an easier and more reliable option, though personally I like to keep plastic items in use as long as possible. Round here damaged butts end up in landfill (or get picked up by hopeful gardeners!). Hope this is useful and good luck.