Encyclopedia of Japan:
suigun
水軍
General term for naval forces in Japan from ancient times to the 1850s. According to the chronicle Nihon shoki (720), Abe no Hirafu led naval expeditions between 658 and 660 to subdue the Ezo tribes of northern Japan. His navy was defeated, however, in 663 at the Battle of Hakusukinoe in Korea. In the Heian period (794－1185) sea forces were maintained by local families along the Inland Sea and northern Kyūshū. They participated in the Taira-Minamoto War (1180－85) and played an important role in the Battle of Dannoura. Naval forces were also mobilized against the Mongol invasions of Japan in the late 13th century. Because of the policy of National Seclusion, daimyō during the Edo period (1600－1868) were prohibited from building large ships. When the Tokugawa shogunate began to modernize its military forces after the mid-1850s, the term kaigun (navy) was used.

worth elaborating to distinguish from the modern term

@@ -12,1 +12,2 @@
-<gloss>navy</gloss>
+<xref type="see" seq="1201330">海軍</xref>
+<gloss>navy (esp. the Japanese navy prior to the 1850s)</gloss>

daijs and daijr have 2 senses, but both seem to translate to 
"navy" unless you want to go into the specifics:
http://ja.wikipedia.org/wiki/水軍