Does the length of a raft change as the stream bed slides by underneath? Consider a raft, of length (distance between its two ends, a.k.a. the distance between "bow" and "stern" of the raft) $L_0$.
Now, the river bed slides by quite close underneath the raft, along the raft axis, at uniform speed $v \gt 0$.
Any two elements of the river bed onto which the raft bow and stern are mapped by a simultaneity projection shall correspondingly have distance $L_{\text{proj}} = L_0 \, \sqrt{ 1 - \frac{ v^2 }{ c^2 } }$ from each other, of course.
Has the length of the raft therefore changed ?

Synopsis of comments (which recently have been "moved to chat", where, unfortunately, they may not be archived for long):
The assignment of a specific length value $L_0$ as "length of the raft" was criticized for "being only true in the raft's rest frame."
In response it has been noted that the relevant identifiable constituents of the raft are indeed supposed to be and to remain at rest wrt. each other, thus being members of the same one inertial frame, and only of this "own" one inertial frame. Therefore, any imaginable statements about "the raft not being in its rest frame" are counterfactual and to be rejected anyways; while the assignment of $L_0$ as "length of the raft (with the raft in its rest frame)" remains applicable.
Additionally it has been noted that $L_{\text{prej}}$, i.e. the distance between any two constituents of the river bed who are identified by one simultaneity projection of raft bow and stern to the constituents of the river bed, can be referred to as "length of the simultaneity projection of the raft to the river bed".
 A: When measured in the frame of the river bed, the raft appears foreshortened in its direction of motion.
The reason is that when considered simultaneously in the frame of the review bed, the two ends of the raft are being considered at different times in the rest frame of the raft, the bow being considered earlier than the stern- that means the position of the bow of the raft is considered first, and then the stern has time to move forward slightly before its position is considered, leading to the length being reduced.
The effect is entirely reciprocal. Suppose people on the raft drop simultaneous markers from each end onto the river bed. In the frame of the bed, the marker at the stern of the raft is dropped first, thus allowing the bow of the raft to move further along before its marker is dropped- the result is that the distance between the two markers in the frame of the bed is greater than the apparent distance between the markers in the frame of the raft (ie it appears to have been foreshortened from the raft's perspective).
Addendum
The raft does not shrink as a consequence of moving, although its length is shorter when measured in a reference frame moving with respect to the raft. If you believe the raft actually shrinks in an absolute sense, then you are quite wrong. The raft is at rest in its own frame. If its length is measured in n different frames at once, each moving at a different speed with respect to the raft, then the raft would have to 'shrink' to n different lengths at the same time, which is clearly nonsense.
Addendum 2
It is not necessary to cite material objects such as rafts and river beds- SR is ultimately a description of space and time in an abstract sense. If you have two points a fixed distance apart in one inertial frame, then the distance between them at any one time in that frame is greater than the distance between them at any one time in any other frame moving with respect to them.
A: The raft does not change its length; neither in consequence of the river bed moving wrt. to the raft, nor in consequence of the raft thereby moving wrt. the river bed (nor in consequence to any other system moving wrt. the raft).
The length value $L_{\text{proj}}$ is a value of distance between certain pairs of constituents of the river bed; namely of pairs which are, pair by pair, identified by simultaneity projection of the raft bow and of raft stern to the (inertial system of the) river bed. Therefore $L_{\text{proj}}$ is, by construction, not the length of the raft, its value is different from the value of the length of the raft, $L_0$, and $L_{\text{proj}}$ should accordingly not be called "length of the raft".
Instead, the length value $L_{\text{proj}}$ is, and can accordingly be called, the "length of the simultaneity projection of the raft into the (inertial system of the) river bed"; or in the given context for short: "the length of the projection of the raft".
