When to use “on” and when to use “in”

The use of prepositions in English is frequently idiomatic. General guidelines exist, but be prepared to learn individual expressions in which the preposition does not adhere to the guidelines.

In the case of the prepositions in and on, here are the most usual uses.

In

in mainly denotes “rest at”:

PLACE: He lives in the country. He lives in Chicago. (BUT, He lives at 2300 Wabash Ave.)
TIME: I’ll be there in an hour.
MANNER: The child ran down the steps in tears.
REFERENCE: In my opinion we need a referendum. They are happy in their marriage.

On

on indicates proximity and position above or outside:

PLACE: He sat on the fence.
TIME: He was not thinking well on that occasion.
REFERENCE: He asked my opinion on the matter.
CONDITION: We’ll hire him on your recommendation.

As for the example about the boat, either is correct, according to what is meant:

The boat is in the water. (As opposed to being on dry land for the winter)
The boat is on the water. (Look at all those boats out there on the water!)

However, it would be unidiomatic to say The ship is in the ocean or in the sea, unless you mean that it has sunk. The ship is on the sea.

Related to the question of when to use in is that of when to use into. While in denotes the state of being “at rest” in a place, or at least being (in a sense) surrounded by something, into denotes motion towards:

The dog jumped into the water.
The children (who were already in the water) jumped in the water.

When deciding whether to use in or into, ask yourself if the person or thing you are talking about is moving from one place to another. If so, use into.

More about prepositions in later posts. Stay tuned!