A folding boat is a magic item that can transform from a small box into a small or large boat and back. If I transform it into a boat and then sail it through an antimagic field, what will happen?
Nothing. The boat becomes a mundane version of whatever shape/state it is currently in (unless the DM says otherwise)
The folding boat is classified as a magic item, despite the fact that its description doesn't indicate that its different configurations are switched or maintained by magic:
This item also has three command words, each requiring you to use an action to speak it.
One command word causes the box to unfold into a boat [...].
The second command word causes the box to unfold into a ship [...].
When the box becomes a vessel, its weight becomes that of a normal vessel its size, and anything that was stored in the box remains in the boat.
The third command word causes the folding boat to fold back into a box [...].
Page 18 of the Sage Advice Compendium lists the criteria for determining if something is magical:
- Is it a magic item?
- Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
- Is it a spell attack?
- Is it fueled by the use of spell slots?
- Does its description say it’s magical?
The first and last apply to the boat. The first, as stated above, obviously true but it doesn't offer any guidance with regard to how the Folding Boat interacts with an Antimagic Field.
The last bullet is also relevant: Nothing in the boat's description says "magic." We know that the boat is magic and therefore its transformation abilities must be magic, but we are left in the lurch with regard to whether or not the box/boat/ship shapes are maintained by magic in some way.
Ryan C. Thompson aptly points out that the boat seems to violate the laws of physics through its capability of containing everything in a 12 x 6 x 6 cube. But then again, D&D is not a physics simulator.
The Folding Boat's description does not give any explicit indication that magic is required to maintain a given shape, only that the transformation between shapes does. This is important because the antimagic field spell says:
Within the sphere, spells can't be cast, summoned creatures disappear, and even magic items become mundane.
Therefore, if we assume that the Folding Boat's magic only drives the shape changing, it is subject to the third part of antimagic field's effect: it becomes a mundane version of its normal magical self. To wit: a box or boat or ship, depending on whichever state it happens to be in.
The only difference between the folding boat being within an antimagic field and outside of one is that the command words that would normally cause it to change would have no effect while it was within the field. It would still function as a mundane boat or ship (or box) while within the field.
Under this interpretation, it's also worth noting that the boat might be capable of being manually converted to a shape (inside or outside of an antimagic field). In other words, inside an antimagic field, it's functionally a mundane collapsible boat/ship kit. Deployed this way, it would be up to the DM to decide if this is possible and how long it takes to assemble or break down since the rules offer no guidance in this matter.
However, a DM might look at the unexpected physics of the Folding Boat and think that magic is required for it to achieve/maintain a given shape. If this is the case, that DM would need to come up with their own explanation for what happens to the ship in an Antimagic field. Perhaps it simply collapses into a pile of parts?
The magic of a folding boat is in the folding and unfolding, not in maintaining a folded or unfolded state.
Whatever state the boat is in when it enters the antimagic field is the state it must remain in while under the field’s effect.
To give some more detail into why, I ask the question: how can such a small box unfold to a large boat? Magic. How can a large boat fold into a small box? Magic. This is evident by the function described in the item description of the folding boat - there are three command words, each causing the folding boat to transform.
Both folding and unfolding are magical effects, so both are impossible inside antimagic field.
Therefore whichever form the folding boat is in persists inside the antimagic field.