Is a jew allowed to go on a boat on shabbos (obviously he/she is not driving). Also what about sitting in a car?

Shalom U'vracha, Even if a Jew is not doing the driving he may not sit in a car or a boat on Shabbos for a few reasons: one reason is that Chachamim said that one may not sit on a boat because he might come to set up a raft (Shulchan Aruch 33:2). He may not ride on an animal or a carriage out of concern that the Jew would pluck a branch in order to help prod the animal. (339:1). Another potential problem with an automobile is that by sitting in one the weight of a person is causing the engine to use more gasoline. Burning gas on Shabbos is clearly prohibited. (see Shemirat Shabbat K'hilchata, (40:83 comment 183, Peninei Halacha-- Shabbat, Harchavot, (9:1). There are more reasons, according to the halacha, that it is forbidden. For a clear and detailed explanation about all of this topic, I recommend that you look in the book Peninei Halacha, Shabbat- 2, 10-12). See there a detailed explanation: https://ph.yhb.org.il/en/01-02-10/ All the best