http://navweaps.com/ - All about late 19th and 20th century naval weapons, and more!

https://www.navsource.net/ - One of the finest pictorial collections for the USN

https://www.naval-history.net/ - An excellent source for all things Royal Navy for WW1 and WW2

https://eugeneleeslover.com/ - If there’s a early 20th manual out there in digital form, it’s probably also here! (and a bunch of older books as well)

https://www.history.navy.mil/ - NHHC has a vast collection of articles, photos and primary sources (I mean, it IS their job!)

https://www.armouredcarriers.com/ - Everything you need to know about the Royal Navy’s armoured aircraft carriers

https://www.usni.org/ - Proceedings, Naval History Magazine, Books, what more could you want?