I do not like the concept of "We take your port, we get your fleet". It is enough punishment to the country who lost the port to lose its fleet, giving it to enemy is a clear game design flaw IMHO, which leads to abuses which are discussed above.
It will be enough if fleet owner country will just not get it respawn, if it sank, until it gets its port back. It may be right to add longer time after a port recapture for gettign fleet back for fleet owner, well, half of hour may be enough (port infrastructure is dammaged, etc.), but NOT giving it to the port capturer country.
Fariz