I'm playing a game as the Aztecs, Emperor, on Continents with Rome to the south, Babylon to the north, and Ottomans on the other continent (to just mention the relevant civs for this post). Rome expanded like wildfire early game and took away any potential for me to expand and have a coastal city. I did early war with them with the intent of razing and reclaiming my coastal territory, but I ended up settling for peace and taking a much stronger food/science city (lots of jungle tiles right as I was building Universities).
All of that about Rome to say that I don't have ANY naval units. Hindsight, I probably should have razed their coastal city then sued for peace, but whatever. Now I'm at war with Babylon and I've dedicated most of my army to taking their capital. The Ottomans, who have the largest army in the game currently, declared war on me. Astonishingly, I have actually seen 1 unit of theirs on my continent. Luckily for me, and perhaps unfortunately for Suleiman, if he wanted to embark all his units and land on my continent, I wouldn't be able to do anything about it at this point. First time I've seen the AI sail the ocean to attack.