I was watching a TV program on Disney the other day, in which an evil scientist decides to flood all the streets in the city with water. The purpose of this nefarious scheme? To make normal road travel impossible, and force everyone to buy his new invention--BO-AT, or Buoyancy Operated Aquatic Transport. I found this quite funny, since the made up full form of the word ‘boat' actually fits in very nicely. But just to be clear the standard English word ‘boat' is not an acronym of a longer phrase; the word actually originates from an ancient verb that meant ‘to break or split.' Perhaps since the earliest boats were made from splitting a tree-trunk and carving into it, the name of the device as we now it today originates from that action. So anyway, I happened to look up the origin of the word because of the TV show, and also ended up reading more about the word. Some of the expressions and idioms associated with the word ‘boat' are quite interesting.
Whatever floats your boat: This is a casual, slang expression, to be used only with friends, ideally. When you say ‘whatever floats your boat' to someone, you are saying, ‘whatever works for you.' Based on context, this could have a variety of connotations though. For example, if a friend is suggesting watching a movie that has failed to be popular, you can agree to watch the movie reluctantly by saying ‘sure, whatever floats your boat.' When you use this expression, you are essentially saying two different things: that you are willing to go along with the plan or suggestion, and that although you may or may not agree, you are keeping your opinion and ideas to yourself. If someone says sleeping all through the weekend is how they like to relax, you might respond: ‘whatever floats your boat.' In this case, you are implying that you can think of better ways to spend weekends, but you are not willing to go into a discussion on the topic right now. You might have realised by now that this expression is somewhat dismissive in tone, which is why it needs to be used carefully.
Rock the boat: As you can imagine, when travelling by boat, keeping the boat steady and stable makes for a pleasant journey. Troubled waters and strong waves can rock the boat though, leading to discomfort. This experience gives us the expression ‘rock the boat.' A politician might say, ‘we don't want this corruption scandal to rock the boat just before the elections.' In this phrase, ‘rock' is used as a verb, in the sense of moving back and forth repeatedly.