Is "International Law" a true law?

Yes. "International law" usually refers to legal agreements--treaties--entered into by governments, on behalf of their citizens. In most countries they theoretically have the force of "actual" law. For instance, the US Constitution stipulates that any treaties that the federal government is a signatory to should be treated with as much import as the Constitution itself. In practice, however, if a country feels that enforcing such a law or treaty would be detrimental to its interest, there is no method (other than economic measures or warfare) to get them to comply.

this question often raises an air of skepticism. international law might not be law to some because it is said not to have an onforcement power. in other word how do we punish offenders? but we must know that international law has enforcement power which often comes in form of sanctions and bans.

I bloody hope so, considering I just dedicated 50+ hours of my life to studying it!

Yes it is although not all countries abide by the laws.

There is International law- These are agreements and conventions signed by countries. Although there are many signatories to most of them, not all enforce them. They exist in theory.

There are laws governing people in international waters they are called the law of the seas.

Civil law does not operate on the persons in ocean going vessels. Only criminal law does.

A popular English saying on this issue goes:

"English Law is law, foreign law is fact, and international law is fiction"

From a German perspective: yes, international law is law. First, we make a difference between private international law (conflict of laws - which are actually national provisions that only regulate the law applicable to international private law cases, such as sales contracts or marriages) and public international law, also known as the "Law of Nations". We then make a difference between public international treaty law and general principles of public international law. While the first has to be transferred into a Federal Law by the German parliament, the latter ranks above all German federal laws, but below the Federal Constitution.

Something else is true for the European Communities: While an EC Directive becomes directly applicable in all member states, an EC regulation forces the member states to enact certain laws.