A new report (pdf)  from the U.S. Office of Naval Intelligence describes Iran's naval order of battle, as well as the Iranian Navy's history, strategic options, and favored tactics.
"Today, Iran's naval forces protect Iranian waters and natural resources, especially Iran's petroleum-related assets and industries. Iranian maritime security operations guard against the smuggling of illegal goods (especially drugs) and immigrants, and protect against the poaching and stealing of fish in territorial waters."
"Additionally, Iran uses its naval forces for political ends such as naval diplomacy and strategic messaging. Most of all, Iranian naval forces are equipped to defend against perceived external threats. Public statements by Iranian leaders indicate that they would consider closing or controlling the Strait of Hormuz if provoked, thereby cutting off almost 30 percent of the world's oil supply."
The unclassified U.S. intelligence assessment was published on the Office of Naval Intelligence website, but last week it was abruptly withdrawn, along with another ONI report on China's navy . A copy of the report was obtained by Secrecy News. See "Iran's Naval Forces: From Guerilla (sic) Warfare to a Modern Naval Strategy ," Fall 2009.