Hi, the two comments above have explained how the phrase originated, but to add to this it is worth knowing that it can be used for situations where there is no physical movement involved (only metaphorical) in the context. For example someone might say " I'm not sure I want to have kids. " and the response (from a person who has children) could be "Come on in! The water is lovely." - meaning that being a parent is a good thing

Naturally, this expression originated from persuading people to join them in a pool/lake or any water to let them know it wasn't too cold. But since then, we often use this expression to simply let someone know that a situation will be comfortable for them. Examples: Sue is reluctant to enter an office because it's full of managers. One of the managers says, "Come on in, Sue! The water's fine." At the party, Mike sees Jane in the kitchen with several friends. He doesn't want to intrude because she seems preoccupied, so he remains at the doorway. "Come on in, the water's fine!" Jane says, inviting him to join them.

It’s typically only said in cases where you invite others to a welcoming environment that they are not familiar. That is, a newly hired co-worker or maybe even a new classmate, perhaps even a guest joining a party at your house for the first time. Personal note: I would probably never use this expression because it sounds too antiquated and too corny for my taste.

Hi Jini :-) It sounds like in that context someone is asking another person to get into the water. Maybe a lake, or the sea? “Come on in” is as you state a coloquial expression you can use in a variety of situations. For example, when someone is outside your apartment door intending to come in, you can tell them: “come on in!” To emphasize that you want them to get in. Is basically, I think, to tell someone to get in somewhere with a bit more enthusiasm. Does this help? Best regards Doriana

Hi, you can use it when encouraging someone to join you in water, whether that is in the sea or a pool for example. It just means that the water is comfortable to be in.