Israel, early in the current activities against Hezbollah. announced an air and sea blockade of Lebanon. For the sea blockade, Israel placed a number of its surface ships (and, presumably, its submarines) in what seeem to be picket positions off the ports of Lebanon. The surface ships provide a visual and physical deterrant to potential blockade runners.
Why would Israel act to blockade Lebanon?
First, let's get "blockade" defined. Here are some cogent thoughts from a Lieutenant Commander C. Troedson, United States Navy:
An offensive blockade is directed toward physically denying the movement of commodities to or from the belligerent nation. The offensive blockade provides strategic leverage for negotiation of disputes by inflicting hardship and inconvenience which weaken the belligerent resolve and works in partnership with a military offensive by cutting off the supply of materials and revenue necessary for continued conduct of war. The naval blockade imposed against Iraq by the United Nations following Iraq's invasion of Kuwait on 02 August l990 is an example of an offensive blockade.What was Israel trying accomplish with its blockade? Two things, in my view, only one of which is specified by LCDR Troedsen: (1) Prevent the removal of the captured Israeli soldiers from Lebanon by sea (and by air) and (2) prevent the resupply of Hezbollah by sea (perhaps from Syria).
The defensive blockade is a protective measure established in order to prevent the movement of unwanted commodities, contraband, or enemy personnel into that nation's territory or to prevent enemy warships from going to sea where they would present a threat to friendly shipping. The introduction of air and naval forces in the "War on Drugs," though not sufficient in numbers to be totally effective, is an attempt to form a defensive blockade to prevent the flow of illegal drugs into the United States. (source)
What form of sea blockade was chosen by Israel? Well, you've got a couple of choices. One choice is the "near blockade" defined by Mr. Troedson as:
The near blockade, off the coast or outside the port of the enemy was popular when weapons were limited in range and the only threat was coastal artillery. In the near blockade enemy activity could be monitored full time. With the advent of mines, torpedoes, and missiles the use of the near blockade became too dangerous and costly to use.As you might guess, if you decide against the "near", you might instead implement a "far" or "distant" blockade, which is one that you hope places you beyond any threat that may be posed by the blockadee. Sometimes, as the experience of the Sa'ar 5 corvette struck by a missile demonstrates, you can only discover the true threats by testing the waters.
However, I doubt that Israel was all that concerned about "near" and "far" blockades. Instead, in deciding where to place their ships they would probably focus on time and distance problems that might be posed by vessels attempting to run the blockade. For, even though Mr.Troedson asserts that "new technologies such as radar and radio communications make distant blockades nearly as effective as near blockades once were," that is only true if your ships can catch what they see on radar or if you have sufficient ships to have one available to vector to a target in a timely manner. If you lack speed or ships, I think that a "distant" blockade, while safer for the assets you do have, will probably not be very effective. Of course, having your blockading ships sunk because they are within enemy weapons range won't help keep the blockade established, either.
Obviously, for an effective balance to be maintained, a fine line needs to be sailed. Underestimate your foe's weapon ranges and you pay a price. Act in an overly cautious manner and you can't set up a really effective blockade.
I assume Israel is using a combination of its corvettte (Saar 5 and Saar 4 in photos) and its patrol boats (Super Dvora Mk III shown) to maintain that line.
UPDATE: Beirut Spring calls it a "useless blockade" here:
Just to be clear, the blockade is NOT intended to prevent the smuggling of the two Israeli soldiers by sea. Had Hezbollah wanted to smuggle the soldiers outside of Lebanon, the easiest way would be to do it through the more-than-happy-to-help Syria.
The blockade has one clear purpose: To make life miserable for the Lebanese in order to exert pressure on Hezbollah.