So here’s the difficult thing with Japanese and prepositions: Japanese does not really have individual words for them. Instead they are conveyed with particles (kind of like a suffix that tells the grammatical purpose of whatever comes before). What complicates it, is that “through” does not have a particle that’s clearly connected with it.
If there is a particle that translate “through” best, then it’s を, but it is used only for transitions (going “through” an area, passing “through” time). For the other sentences the translation does not really have a part meaning "through.
I looked through the window.
窓から見た。
This is probably what normally would be the meaning of this sentence, but the Japanese literally means “[I] looked from [the] window”, with the particle から marking the point of origin. If you want to be more explicit about the looking through part, you could say:
窓越しに見える。
Here 越し is the nominalisation of the verb 越す: to pass through, so 窓越し is “passing through the window”. 見える means to be visible, so this sentence literally means “[It] is visible [in the way of] passing through the window.”
The oar slid through the water.
オールは水を切った。
If we were to literally translate the English sentence it would be オールは水を滑った, “The oar is gliding through the water”, but this would be very weird. Instead, we say that the oar “cuts through” the water. However, the verb 切る (to cut) is transitive, so を not really means “through” here, because it is not used as a transition: in this sentence を marks the direct object of 切る; the water is the thing that is being cut.
The elephant smashed through the spear-wall.
象は前線部隊を破った。
象は前線部隊を突破した。
I have no idea what a spear-wall would be in Japanese, so 前線部隊 means “frontline” instead. The elephant (象) breaks (破った or 突破した) the front line.
The procession passed through the streets.
行進した。
Now this one was the weirdest, since a procession already passes through streets kind of by definition of what a procession is: the noun 行進 that means “parade” is also used for the verb 行進する “to parade”. Of course we could use a different verb to mean parading / passing through a street, but it all becomes kind of superfluous (which in Japanese means it should be left out). Hence, after the translation, the only thing left is 行進した; “[it] paraded”
The flour filtered through the sieve.
小麦がざるを通った。
小麦 is flour, and ざる is sieve. The verb 通る means to go through. So this sentence is kind of basic. Here the particle を is used to mark a transition, since 通る is intransitive.