Hmmmm, sure, a submarine to explore the near-frozen hydrocarbon lakes on Titan. Oh, and do it by the 2030's. What was NASA smoking when they proposed this? Nothing at NASA happens that fast. So right off the bat, this entire project should be ignored, because it lacks credibility. That said, first priorities for NASA should be to spend 10 yrs living on the Moon, developing technologies that'll enable the first Mars colony, then, and only then, send people to Mars and let that be the top focus. Second, choose the best icy moon and first put a sophisticated probe in permanent orbit around that icy moon and study it 24/7/365 so we can understand exactly what it'll take to get under the ice and explore the murky depths below. Sheesh. This isn't the county fair where you throw a dart at the balloons and randomly get a prize. Prioritize and plan.