What would be the dangers of entering/falling or navigating the Sea of Shadows prior the 3rd Maelstom?
Also, a vampire that could fall into the Sea of Shadows would suffer what kind of pain/damage (mentally and physically)?
I didn't find an official answer but here's how I would homebrew it.
Every hour take 1 aggravated damage. If the player runs out of corpus they have to fight their shadow but the shadow gets 3 additional dots to spend (the storyteller could make it be for only that fight or forever). You could justify the player waking up in oblivion if they win but that is up to the storyteller. Specters would also attack the player.
You could change the dots to angst if you wanted to.
Since the sea of sorrows is described as being "cold fire" you could reasonably justify running it as either cold damage or fire damage. I personally would use cold so that wraiths would fall faster than vampires. If you wanted to include mental damage I would subtract 2 from rolls to resist raging. Specters would still try to attack.