Departed Deckhand

When this creature becomes the target of a spell, sacrifice it.
This creature can't be blocked except by Spirits.
{3}{U}: Another target creature you control can't be blocked this turn except by Spirits.

The Lost Caverns of Ixalan Commander

When Departed Deckhand becomes the target of a spell, sacrifice it.
Departed Deckhand can't be blocked except by Spirits.
{3}{U}: Another target creature you control can't be blocked this turn except by Spirits.

Jumpstart

Core Set 2019

Activating Departed Deckhand's last ability after a creature has become blocked by a non-Spirit creature won't cause that creature to become unblocked.

If the spell that targets Departed Deckhand has no other targets, it won't resolve (because it no longer has a legal target after Departed Deckhand is sacrificed).

You'll sacrifice Departed Deckhand even if you counter the spell that targets it.