The Fire Nation ships are based on the design of the IJN ships in WW2, particularly the Yamato. For the Case of the Royal Sloop, I find it funny that they gave it a literal pagoda mast as this design feature on Japanese ships in WW2 has been the source of much ridicule and mockery due to them being so awkwardly and comically tall. The most notorious example of such mockery being towards the Fuso class battleships.
To be fair, the pagoda mast actually looks like it fits the ships in ATLA.