If it is used for drinking water, be carefull what glues you use. Must be FOOD SAFE glue. I dont think you will find anything food-safe that will also adhere. If it is just a smallish hole, could you place a stainless steel BOLT through it and a foodsafe rubber washer then tighten it. Or re-purpose the tank for a non-drinkingwater use.
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Check carefully; the majority of large water tanks I've seen are polyETHYLENE, PE (or polypropylene, PP) and not polyurethane. Look for a recycle logo and check the number. If it's polyethylene it is extremely difficult to glue; most glues do not adhere well to PE even after surface treatment. Heat-welding may give a reasonable repair. Harbor Freight sells a plastic-welding kit for about $40, but it takes some practice to be able to rely on the welds made. You may want to check the phone book on 'plastic repair' or similar, and find someone who knows what they're doing to repair it. Good luck!
I've used heat, in the form of a hot piece of metal to seal crack in polyurethane surfaces, , but it's probably not optimum. I think there's a poly goo you can use to patch them, but I don't know what its name is or where to get it.. I suppose one could use fiberglass impregnated with poly, although I'm not sure what the cure rate or best method would be. Silicone might be a solution too, but I suspect someone is going to say "that's the last thing you should do!" Sorry, lame answer, I know, but I'm sure someone else will come up with something netter. just being a busybody.