I am not licensed to practice law in New Jersey so the following should not be taken as legal advice, but simply as information based on general principles of law which is intended to educate. If you need legal advice, please consult a lawyer who holds New Jersey licensure.
You MAY have a claim against the seller of gas; but remember, as plaintiff, the burden is on YOU to PROVE your case "more likely than not." That may be hard to do, especially if you don't have the gas which was removed from your engine. You can call up the gas station and put them on notice of the issue and tell them you want reimbursement. However, if there was gas with water put in your tank, you're not the only one. If you're the only one, chances are the water got in there some other way, or at least chances are the small claims judges will not be convinced.
Remember, "preponderance of the evidence" means "more likely than not." If the evidence is evenly balanced, you lose.