Now we know the real reason they pretty much always sailed within sight of land: they were looking for a good place to plant their next meals!
Ancient ships didn’t hug the coastline, it’s asking for trouble to do that. But it’s easy to see that a first-time voyage to map and explore would have to take those risks, and of course, would make slow progress.
Probably (imho) most of the surviving ancient wrecks and wreck sites (sites where just the durable cargo remains, the wood etc being long gone) will be found in the abyssal plains of the seas and oceans. Hardly anyone has bothered to look because it’s expensive.
Late in the 1990s, Robert Ballard used the NR-1 (a US Navy sub that is rented out for research now) to look for an Israeli sub that went down in 1960. In the process of looking for it, he located three other wrecks, at least one of which was ancient (sez here 8th c BC) circa 400 meters depth. I think the NR-1’s dive limit is about twice that depth, but I’d have to look that up, and I’m lay zee.
Here’s the link (which I didn’t check to see if it’s still live) for the Ballard thing:
http://web.mit.edu/deeparch/www/publications/papers/BallardEtAl2002.pdf