Sorry for the ship/boat confusion. But I think a size small octopus would not be likely to grapple a fishing boat either... and its size was not apparant until it drew nearer. Remember, you guys are backlit by the moon and traveling in a white vessel, while the approaching vessel has the mainland behind it.
The rule of thumb for the ship/boat distinction is that a boat can fit in a ship, but a ship can't fit in a boat. So, yeah, it could be called a ship, however...
A Cromster is about the size of a Sloop, which is commonly considered a boat. It has a draft of 2 fathoms (it needs 12 feet of water to avoid running aground), and cargo capacity of 90 tons, whereas a Sloop (also a 2 fathom draft) has a cargo capacity of 80tons. A Cromster is less maneuverable than a Sloop, though more heavily armed and with a slightly larger cargo capacity.
Compared to what you were traveling in before, it's a pretty small ship though. And there are larger ones in the fleet you were accompanying. The HMS Sea Stag, a Third Rate, has a draft of 4 fathoms (meaning it needs ~24 feet of water depth to avoid running aground) and a cargo capacity of 120 tons, which is low for its size because it's not fitted to haul cargo. The flag ship, a First Rate, has a draft of 5 fathoms and a cargo capacity of 200 tons (also not built for hauling cargo... built for blowing stuff up). A galleon, which is smaller than a First Rate, has a cargo capacity of 400 tons.
The speed of these ships is a lot faster than that given in the DMG, but wind conditions are not ideal. I am using the Corsair
rules for ships because this is a port city and the DMG rules are inadequate. Stormwrack has a few rules, but doesn't really suffice.
I guess I also called it a boat because I tend to include larger vessels in what I call boats... my dad had a boat that was 30' long. I was raised on boats. I think an octopus would have a hard time grappling a good sized rowboat, really.
Boats are big. Ships are bigger. Yes, a Cromster is a ship, though not a big one, but I doubt a size small octopus, fiendish or not, could grapple anything larger than a canoe.