In another Reddit thread, u/Detzeb pointed out the significance of Jonah's question. It is not just a random hypothetical, but actually foreshadows what happens to Erin's mother, Helen. The mention of a firing squad represents the actual murder method the cartel ends up using to kill Helen: They shoot her. The mention of drowning represents water torture methods the cartel uses to extract information from Helen. Jonah's question attempts to reveal the danger Erin's mother is in without discussing the topic directly.
Sadly, Jonah's words are lost on Erin at the time. After Jonah asks the question, Charlotte and Erin both screw up their faces in disgust, and Charlotte says, "Jonah, stop acting insane." Just then, some teenagers come to pick up Erin and Charlotte for a party, leaving Jonah alone, and the scene ends.
Jonah's question represents his desire to tell the truth and act morally, which is being inhibited by his parents' rules. He feels conflicted because his value of honesty is currently in opposition to his value of family loyalty (both of which are part of why he's one of the most likable main characters on "Ozark"). Unfortunately, Erin's reaction to Jonah's question represents how she is blinded by her sense of what is normal and is thus unable to see the truth about what is happening right under her nose.
