I have special events on (even though the majority of them are ridiculously underpowered for RoM, and many of the rest consist of "Oops, sorry, there goes that 50-turn-build!" with no option to save it - tho of course I keep 100 gold on hand at all times for those with an option. Lots of creative space left in events, let's say) to keep pre-Astronomy vessels where they belong and to preserve some land for a later Deity colonization race.
If my poor Triremes accidentally happen onto an ocean tile even in my own borders(!), they invariably hit Davy Jones Locker and this somehow causes a storm of three millenia duration. Going into world builder to try to fix a CTD, I notice that Ashurbanipal has somehow colonized a continent 10 ocean tiles from his own using only War Galleys and Galleys. Gilgamesh too has a city on that continent and managed to circumnavigate the earth around 2000 BC. Now I've just gotten a visit from a BtS deathstack conveyed by a convoy of 20 Assyrian Galleys who braved a 12 tile voyage on the high seas.
Pls include an option in 2.8 that allows choosing no ocean movement for ancient vessels. The current system is non-functional.