lets say you still lost that one, you would have the zombie killed so....
roll a 4 on the next round
add up 1 crew (+1)
add up cans (still +1)
add up spells and bonus' (+0, no more zombies in the lead)
so a total of 6.
you can add the cannon bonus only in first round of boarding and regarding the undead crew point, your crews will be mostly zombies, so the -1 is still there... But the principle is right.
Just an example how the Undead crew works:
If you take the Liche (best Undead wizard costing 300pts!!), you have 15 crew points per 500 points.
So you have 30 crew points in a 1000 point game. Zombie crew is 1poit and -1 in boarding, Skeleton cres is 2 crew points and no bonus in boarding, Wight is 3 crew point and +1 to boarding action.
So the list would be:
Liche 300
Floating necropois 250 (8 crew slots)
Ghost ship 150 (5 crew slots)
Death ship sq 150 (total 9 crew slots)
Death ship sq 150 (total 9 crew slots)
that is 8 ships with 31 crew slots. So even vith 300 pts wizard and only 8 ships, you are not able to fully men your ships and all the crew would be zombies with -1 for boarding.
If you take Tombk king as admiral instead of Liche, you would have more points on ships, but only 20 crew points! I really don't know what were the "designers" thinking....