What does swim up mean? To "swim", for example, in a river, or go through a canal "by ship"etc?

English is a language of context. A word can have many meanings as both literal and as an idiom and change in word combinations. Even a full sentence can be ambiguous with two meanings.

"Swim" as a word is a verb meaning to propel oneself in water. Purposeful motion to water

"up" is a direction, either against the flow of water, or to the surface from below the surface.

Swim up, literal use, is propel in water against the flow of water.

Swim is used in idioms.

https://idioms.thefreedictionary.com/swim

"swimming with the fishes" = gangster criminal expression, To be murdered and have one's body disposed of in a river, lake, or ocean. (A less common variant of "be sleeping with the fishes.")

"Swim upstream" = doing something a more difficult way, or against general opinion

"sink or swim" = fail or succeed as a project or activity

Generally, "swim" can be used as a replacement verb of motion or activity with or against a general opinion or direction.

Ask again in a full sentence to get a more exact meaning.

"Swim up" refers to swimming in something that has a current in it, such as a river, and swimming so that you are going against current.