Looking on the internet deeply has found these results:

The boat is on the river. is the most popular phrase on the web.

The boat is in the river. 13 results on the web	The boat is on the river. 19 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
The boat on the Ohio River. The cat's in the bag... and the bag is in the river. Some of them were flown here to the river... and came all the way up on the river in boats. That's it, I'm going to go watch the boats on the river. From there we'll take a boat up the river. My cousin Earl got the luckiest boat on the river. They like to play the slots at The River Boat. Enclosed as a PDF attachment, a picture of yourself, in a boat, on a river.	My cousin Earl got the luckiest boat on the river. The boat on the Ohio River. Some of them were flown here to the river... and came all the way up on the river in boats. The back is on the river. From there we'll take a boat up the river. They like to play the slots at The River Boat. That's it, I'm going to go watch the boats on the river. A boat awaits you on the river.

The boat is in the river.

13 results on the web

The boat is on the river.

19 results on the web

More popular!

The boat on the Ohio River.
The cat's in the bag... and the bag is in the river.
Some of them were flown here to the river... and came all the way up on the river in boats.
That's it, I'm going to go watch the boats on the river.
From there we'll take a boat up the river.
My cousin Earl got the luckiest boat on the river.
They like to play the slots at The River Boat.
Enclosed as a PDF attachment, a picture of yourself, in a boat, on a river.

My cousin Earl got the luckiest boat on the river.
The boat on the Ohio River.
Some of them were flown here to the river... and came all the way up on the river in boats.
The back is on the river.
From there we'll take a boat up the river.
They like to play the slots at The River Boat.
That's it, I'm going to go watch the boats on the river.
A boat awaits you on the river.