An interesting point here is that if the US is considering some kind of potential future naval war then it's going to have to fortify the coastline. This is an extremely expensive job of work, because you're talking about fortifying all the entrance places to a continent.
The Endicott report identified 22 ports on the East or Gulf coasts as needing protection:
1. Kennebec River, ME
2. Portland, ME
3. Portsmouth, NH
4. Boston, MA
5. New Bedford, MA
6. Narragansett Bay, RI
7. Eastern entrance to Long Island Sound.
8. New York, NY
9. Delaware River
10. Baltimore, MD
11. Washington, DC
12. Hampton Roads, VA
13. Cape Fear River, NC
14. Charleston, SC
15. Port Royal, SC
16. Savannah, GA
17. Key West, FL
18. Tampa Bay, FL
19. Pensacola, FL
20. Mobile, AL
21. New Orleans, LA
22. Galveston, TX
Not all of these are perhaps vital, but most of them are significant. They will need defences capable of standing off a raid by at least a couple of modern ironclads.
The monitors are better than nothing, but one by itself is just a good way to offer a practical demonstration of the vices of cast iron laminate armour - you'll need to do a lot of building or a lot of fortifying. These works will also need somewhat regular modernization (no good that you just spent a lot of money on 15" Rodman guns, the "threat" is now ships with rifles, so better build a few hundred heavy rifles), and will be costly of land too.
You also need defences for e.g. San Francisco on the West Coast.
One plus side of this is that it means the Army won't be complaining about not getting enough funds. One minus side is that you're going to need a much bigger army.