So, in the Ghosts of Saltmarsh adventure book, it gives more in depth information on sea adventuring and ship information. I am running a campaign and my players are located on a large salt water lake (think Lake Erie/Ontario size) that is attached to the sea through an inlet that allows larger sea vessels access to the lake.
They have just recently got access to a folding boat. My question is whether the folding boat can really be thought of as a seaworthy vessel. It folds out to a 24 ft. long boat with a single sail and 5 rows of oars. In the Ghosts of Saltmarsh supplement, the smallest "sailing ship" they note is the Keelboat, which is more than twice the size of the folding boat's largest size (60ft vs 24 ft).
This makes me think that the folding boat would not be seaworthy. And possibly not even good to take too far from shore on the lake if the weather is particularly bad.
As the DM, should I rule that the folding boat will be fine for travel on the lake but that, if they want to go out to sea, they should either charter or even purchase a larger vessel?
The description implies it's safe to use
The description says nothing of how sea-worthy it is in terms of bluewater sailing, so it ultimately comes down to your decision. But the fact that there are two options of boat styles suggests different uses.
The Folding Boat is not a big vessel.
This is a pretty small boat with a very shallow draft. Combining those factors along with the lateen sail suggests that it's made for staying closer to shore. Lateens are best used in that situation where you're not sailing with the wind directly behind and prefer the maneuverability that it provides.
This is a larger boat, and in modern days there are deepwater open ocean vessels that are about this size. Combining that the ship has a deck and utilizes a square sail suggest that this is possible for open ocean sailing.
How storm-worthy it is up to you
However, Boat 2 is still fairly small. It's going to be slower than other deepwater, open ocean vessels and there may be issues if they run afoul of inclement weather.
Who is sailing it?
The bigger issue here may not be in what type of boat, but in who is proficient in it's use. If your players have that (or can hire), then I think it's fine to give them the opportunity to use it. It doesn't have a lot of other uses :)