Hello, we do have the same problem but in June. There is a big gap between 6.30 and 23.45:-( I am a bit disapointed cause I did believed there will be more ferries in Summer.. BUT: I know there is a ferry to Kalymnos from the main port (Kos) I think at 21.30. Check the blue stars ferries.. We have never used the ferry from the main port (always coming from Mastichari) so I do not know about the taxi prices (should be more expensive- it is about 25 kilometers away from the port). Please let me know after you manage the trip (how did you go) Thanks!