Strangely, I seem to be unable to reach the Eel-Infested Waters region. I've tried both of the boats at Silver Cove (the one near the town and the one on the island farther north) on all seven days of the week, and neither one seems to go there.
According to an FAQ I found, the northern boat (the "Barracuda") is supposed to travel there on Wednesdays, but every time I go on Wednesday, the captain says "Sorry, come back tomorrow".
Is there some quest I need to perform to make Alamos reachable?

...and before that point, no boats leave for Alamos? Okay, then. Thank you!

You're welcome.
And no, you can't go there before that point. It's the same with Hermit's Isle.

Necroing a very old thread, but I recently also ran into the problem that I couldn't get to the Eel Infested Waters region from Silver Cove. So if anyone else is running into this problem, here's my experience.

Apparently it's different for different people, but for me (version mm6_2.1.0.42 on GoG, with GrayFace patch for mouselook and such), I only get the the prompt to travel to the North Isle from Silver Cove on Wednesdays, and it takes me to the north isle in the silver cove region. The boat on the north isle always says it's going to the south isle, but actually it goes to the Eel Infested Waters on Thurdays. The travel time is also 2 days on Thursday, instead of one day on the other days of the week.

And I just tried it, but without doing anything for the main quest, I can go to the Eel Infested Waters from Silver Cove, as long as I take the boat from the northern island on thursday. The status of the main quest changes nothing, here.

Indeed. henkie is correct. DrIstvaan is wrong.
You don't need ANY quest or story progression to explore the entire world map.

The special boat that takes you to Hermits isle IS story driven, but you can go there by foot anytime.

Only certain dungeons are locked until you progress.

Castle Alamos is on "Island North" in Eel Infested Waters. Ships sail there from Silver Cove town on Wednesdays (in my version of the game) and from "Island South" in Eel Infested Waters on Saturdays. Silver Cove's northeast island is called White Rock (according to a random NPC convo). The ship Barracuda sails from there to Island South on Thursdays, and Saturday is when it arrives.

Has a later version of Grayface patch "corrected" the Wednesday sailing from Silver Cove town to make it go to White Rock island, 1 day voyage, to connect with Barracuda's sailing?

I don't know. I have never used a boat to travel to another nearby destination on the same map I'm already in.
Even with parties without the ability to cast walk on water, I just either hire a water master NPC, or have my cleric/paladin use a water walk scroll.