So, a weak naval explorer unit that can't carry units or only carry scouts is in order? Galleys should be restricted but another unit serving as a naval scout should be available if you want to take the risk?
Absolutely not!
That's not really an argument against allowing a very, very small chance of success, though. Add in a nice supply system to make the upkeep of a long voyage very high, combine it with the exceptionally low chance of success, and you have a situation in which it is very unlikely in which galleys can traverse oceans, but it is still a remote possibility.
Ceder was available and used, and isn't so porous. It would just be a better than average vessel due to the cost of ceder.
But in reality it isn't even a remote possibility. It's simply physically impossible - for a galley, anyway.
There are other classes of ancient vessels that have a possibility (a kayak or catamaran being the best bet), but not galleys. They can't stay in the water more than a couple of days at a time, even if you had a magical extra-dimensional space in which to store enough supplies for 200 rowers. They'd become waterlogged in just a few days, and turn into a submerged (but still floating) wreck, because of the types of wood that had to be used in their design. You're as likely to cross the Atlantic in a galley as you are to cross on a tree that fell in the water.
In the game, I think 'galley' is meant to represent 'ancient boat' more so than what is actually technically a galley. And it is seemingly possible for rudimentary boats to cross large ocean gaps. Hence why the Pacific islands were populated.
The boats used were not at all 'rudimentary'. They were small ... but that's not the same thing as rudimentary. We don't use galleys or caravels anymore, but we do use catamarans - not just for recreation, either. The hull design is widely employed in ferries.
They were produced by a society that was intensely specialized in sea travel, and capable of incredible feats of ocean voyaging. They are exceptional.
Well, I was using 'rudimentary' in the sense of 'ancient' (admittedly not the correct sense), not 'basic', with my point being that it should be somehow possible to enter ocean tiles before caravels, even if the official designation of the naval units used to do so is 'galley'.
with my point being that it should be somehow possible to enter ocean tiles before caravels
Why? It makes it interesting.