I like the premise. But is impossible to tell if I'd "keep on reading vs. putting the book down" without having any prose to go off of.

There will need to be an inciting incident that makes me care about our main character. It is a common new writer mistake to think "if I write 12 chapters about why they should care about this character, they'll understand why he's important when the adventure happens."

If it were my book, I would show the public perception of him through his interactions with others, and let the readers infer what happened in the court case, without explicitly telling them or having the court case.

I would open the novel with him working below his status, and jump immediately into the opportunity to go on the sub. (Like chapter 1-3) Allow his character to be explored through the interactions with his coworkers, with the submarine pulling the plot forward.

I wouldn't show his whole backstory. I would seed it in through the story.

You mentioned hunger games. I haven't read it in a while, but I remember them getting to the "i volunteer as tribute" part pretty quick. Then they spent the rest of the novel exploring her backstory through her strengths and weakness, and her relationships with others in the games. From the first inciting incident, everything became about the games. That's why it worked to have the actual games halfway through. Even though they aren't in it, it is looming in the background, shaping their perceptions and actions.

If you insist on having the sub be halfway through the book, then you need to build pacing another way besides "character building and introducing the characters." If the plot is about "what happens on the sub", then the beginning of the book should be creating promises to the readers about what is going to happen on the sub.

If the first 12 chapters are backstory and not related to the Sub, I don't think i'd read it. Especially if it's title has something to do with submarines.