Whoa. Time out. Your daughter is only a sophomore? She has plenty of time left to bring up SAT/ACT scores and let the process play out. Most sophomores haven't even thought about taking the SAT or ACT. A lot of improvement will come with maturity as well. So, don't sweat it at this point. Besides, she can work on her math skills and bring that score up over the next 18 months. If you push her too hard, or she pushes herself too hard, it will be counterproductive.
Now - on to other issues - I'm a Blue and Gold Officer for USNA and a Field Admissions Rep for USMMA. So, I have some insight on the process and what the academies are looking for. I'm an '87 USNA grad and retired Marine reserve officer. My oldest daughter is USMMA class of 2020 - wondering if there will even be a graduation ceremony. Yet, she will graduate and be a 3rd mate/deck officer. She plans to sail on her license and absolutely loves KP (as much as anyone can love a service academy).
Academics are an absolute killer at USMMA. I mean - harder than any other school. It might be the easiest SA to get into, but the hardest to graduate from. You have 10 week trimesters to get in what most schools do in 16 weeks. And then there's sea year with two sailing periods (4 months and 8 months) with sea projects that you must stay on top of daily while at sea or you fall too far behind. My daughter's class started with 279 and they're down to 210 - mostly lost due to academic failures. There's no way to catch up if you fail a course, so often times (if the academy wants to keep you) you do a setback to the year behind you and pick up there after a year at home in a local college making up work. There are no summers off (at least at USNA, there will be some summer leave time - about 4 weeks- off). 75% of every class, or more, will sail on their license and take a reserve commission in the Navy. About 15% to 25% commission active duty in the Navy or another branch. USMMA is a trade school that trains you to be a merchant mariner - that's the goal and the mission. The mission is not to commission active duty, though some do as I've stated.
Circling back - Sea Cadets is a great program, but is not weighed heavily at either USNA or USMMA. It just isn't. The schools would rather see the candidate taking more AP courses. But it does offer leadership opportunities if she's not doing sports. If she is doing sports, then focusing on becoming a team captain is a must. And she needs to be involved in other activities like clubs and do a TON of volunteer work - I mean a ton. Community service is weighed heavily these days.
Don't discount NROTC. NROTC is a fantastic way to gain a commission.
Do not think of going to a SA as a way to save on college tuition. That should never be the reason for going to a SA. Never. There is plenty of money out there - especially at small, private colleges regardless of your family situation and ability to pay. If that turns out to be the way she ends up going, that's fine.
Some other points I want to make regarding your original post:
1. Not everyone with high SATs is just looking for a free tuition school. Many, if not all, of those I work with are interested in going because they want to serve.
2. It isn't true that 35% leave. It's just not true. Not sure where you got that info from. USMMA may have the highest attrition, but it's because of academics getting the midshipman and not because they leave on their own accord. USNA has a very high graduation rate - nearly 90%.
3. And just because you think your kid wouldn't quit, doesn't mean that he/she wouldn't. I've seen it happen with those who were 100% all in decide after the summer (plebe summer or Indoc) or after plebe year that they just don't want to be at a SA. It happens. And if it were to happen to your daughter, you would need to support her and not make her feel like a failure.
4. Despite all the hard work, it's a frequent occurrence that a kid is qualified academically, physically, and has a nomination, but there's a medical condition that isn't waiverable that keeps the kid out of a SA. It happens. Be prepared in case it does.
So, encourage her, but make sure the dream of going to a SA is hers and hers alone. You're not the one that's going to be going through the academic, mental, and physical rigor. She will be. Since she's only a sophomore, a lot can change between now and when she applies and/or gets accepted.
Good luck to your daughter and I hope she makes it. But the road to get there is long - especially since she's only a sophomore.