If your bun is dehydrated, it would be better to give electrolytes. If you have Pedialyte, or something similar where you are, then that is the best thing to use. It's an electolyte drink for children, and you would want the plain unflavored kind. If you don't have this available, you can mix up your own, but it's not as good as the pedialyte. You can also sometimes get electrolyte mixes used for livestock, at farm or feed supply stores.
You should only use this homemade mix if really necessary, because sugar isn't the best thing for rabbits, and can upset their digestive flora. If your rabbit is eating, and just isn't drinking enough, then syringing plain water is just fine. If your rabbit is severely dehydrated, it would be better to see the vet and get sub q fluids given.
Home Made Lactated Ringers
Electrolyte solution for re-hydration
Mix the following in a jar:
8 oz. of warm water
½ Tablespoon of sugar
1/8 teaspoon of salt
1/8 teaspoon of baking soda
Still well, and refrigerate. This solution is good for 2-3 days when mixed