Rav Shlomo Zalman Auerbach: Discussing Your Overseas Trip On Shabbos
Any action that you cannot perform on Shabbos, you may not discuss on Shabbos taking this action after Shabbos. So for example on may not say that he is traveling by car tomorrow since he may not drive a car on Shabbos. However if he only says that he will travel to a certain place but does not mention how he plans to get there, even if he would clearly need a car or some other form of transportation to get there, it is permissible to discuss since he did not explicitly mention the actual Milacha and only hinted it.
Even if the destination is outside of Tchum Shabbos he may say that he is going there, since it is possible that there are small huts along the entire way that extend the Tchum Shabbos. However Rav Shlomo Zalman Auerbach says that if your destination is overseas, this logic would not work and it is assur to discuss, even if you don't explicitly mention that you are going by plane, since this is more than obvious and is considered like you said you are flying even if you didn't.
However, says the Orchos Shabbos (2:22:13), it is possible to be Matir based on the Aruch HaShulchan who says that you may talk about a sea voyage on Shabbos since under certain circumstances it is permissible to embark on a sea voyage on Shabbos and you can get to your overseas destination by boat as well.
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.