The structural stuff is well covered above, so just a player-facing angle: the shared-move premise is a nice clean pitch, and it's lovely that you show the optimal route after a clear. The one thing several replies converge on is that harbour 1 needs the shore trick while its tip only says the boats move together. I'd make harbour 1's tip literally say "park one boat against the edge," and maybe flash the held boat briefly when it doesn't move — that visual cue would also soften the "opposite arrow isn't an undo" surprise.
Happy to swap notes if you like strategy on mobile: https://deunastudio.itch.io/beacon-breakers-merge-defense