A few quick comments on possible tactics that can be utilised. A "persistent-stare" capability will be very useful in this scenario. This is normally done via the use of unmanned aerial vehicles (UAV). The next possible capability desired will be a method to insert a boarding team covertly. Most likely to be done at night, a submersible can deliver boarding teams to overpower the pirates. One issue could be the carrying capacity (or lack of) of the submersible to carry enough troops to outnumber the number of pirates on the yacht.
On the matter of the waters near Somalia being a pirate hotspot, opswarfare recently came across a UN mapping website which incidentally has a detailed map showing how bad the situation is.
The file (PDF) is available on this page.
France has some advantage over other Western countries in dealing with this issue, as it has permanent military assets based in Africa, e.g. troops, aircraft, vehicles etc. This will stand it in good stead in the scenario that they are now facing. In this case, the nearest French presence is in Djibouti, with the 13e demi brigade de légion étrangère (13e DBLE) (13th Half-Brigade) of the French Foreign Legion based there.
UPDATE (11th April 2008): It seems the crew has been released. No further details at the moment.
UPDATE (12th April 2008): The plot thickens. It seems that ransom was paid and the crew were then released. After that, the French forces pursued the pirates ashore. Some of the pirates have been taken into custody. Part of the ransom was also recovered. 2 reports seen so far (Washington Post, Independent).
If the reports are true, then kudos to the French for ensuring the safety of the crew, and also discouraging further attacks with its strong action of apprehending some of the pirates involved.