Answers are in someway connected to water,rivers, bodies of water, fact or fiction, sayings, songs, people and novels.
30. Drakes last resting place (6,2,4,3)
31. A final, decisive defeat (8)
38. The Turkish Oxford! (3,9)
43. Lake? - No! (5,5,)
45. Indication of indifference (5,3,1,5,4)
50. Final trip for firewood! (4)
The etymology of the word Bosphorus comes from the Greek mythology, Era chasing Io, for his idyll with Zeus, transformed her into a cow and to avoid the wrath of the goddess, the poor animal was forced to cross the strait which took the name of Bosphorus : The ford of the cow (bos =cow ; phorus = ford) like Oxford in England.
...from Brewer. Zeus, enamoured of Io, changed her into a white heifer from fear of Hera, to flee from whom Io swam across the strait, which was thence called "boos poros" or 'ford of the cow'