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If I am not mistaken, there is a bus which goes to Kotor at 19:00 (I got that from the website of Dubrovnik's bus station).
I've found also posts (in Spanish) of people recommending to take the ferry between Kamenari and Lepetani to cross the bay, but you would have to get there from Dubrovnik, I don't know if there is any bus connecting Dubrovnik and Kamenari. This is the ferry website
Also take a look at this to check the different possibilities that you have to get there.