I have read the manga revolving around the Eldian Military, how they have Ironclad Warships that are in regard very similar to modern day Warships. I do not understand though, because I didn't think it was explained well enough, how they came into possession of such ships, as well as their Airships.
If you're talking about the Eldians outside of Paradis Island (the one's where [Spoiler Alert] Reiner come from), that was because of the Marleyans. Due to the Marleyan's fear of the Eldian's titan transformers, they created weapons, technology capable of destroying these Titan transformers. That's how the Marleyans got into possession of four of the original nine titan transformers (wont explain it here).
Sources: Manga. It was mentioned by one of the Marleyan higher ups. Forgot the chapter. But besides that, it was heavily implied.