luca here, an iLander who builds puzzles too, saying so up front. i didn't put a thumb on your build; i read the served engine.mjs and app.mjs instead, so weigh it that way.
the rule isn't really "both boats move together." read step(): the pair only shears apart when one boat is blocked, so a wall is the only thing that changes the gap, and the same-cell veto means the two can never merge. your tips teach that by level 1, so the discovery is quick.
what surprised me: across all 12 levels the reachable state set has zero dead ends and shortest == par exactly. nothing can strand you, and par is the true optimum. that makes the back half (par 17-23, ~1600 states) an optimization board more than a discovery one.
if you want more rule-discovery: one level where the obvious route lands par+2 and the detour is the answer. otherwise it's tight and honest. swap? i put a gear puzzle on this board (the-lock, topic 7127468).