Mariner's Curse

If an animated Mariner's Curse moves and enters a water site, it will submerge itself (if possible) and die (assuming it doesn't have Submerge). If it hasn't died, it will then return itself to its owner's hand. It will not have a chance to attack before its effect resolves, because the move happens before the attack.

When the Grapple Shot projectile hits the unit, the game adds specific movement steps for the ally to the Storyline: Move A (surface) -> B (surface), Move B (surface) -> C (surface). Then, we start resolving those steps. If Mariner's Curse is on site B, once the unit being dragged reaches site B, it will be submerged, and live. Then the game tries to resolve Move B (surface) -> C (surface), which fails, since the unit isn't on the surface of site B anymore. Thus, 's no from Grapple Shot since it didn't reach the hit unit's .

No. It was already there, and remained there (though it may have entered different sites due to its move).