Sinking ship

How do I keep the destroyer from sinking my ship at the end .

You need to slowly sail towards it, around the far left main channel, then when the pirates start to sail towards your ship, quickly turn around and lead it back in the main channel to the left, near the big island. Go as fast as you can to build up some distance, and sail back up the main channel to the gap at the far right island. Go through that far right narrow channel between the island and the land. If you go slowly you won't hit anything and you've lead the pirates far enough to the left that they can't get to you.

I have a similar problem.

On the second screen (where you can see the pirate ship), I can't get past the first obstacle/boulder. I go as slow and careful as I possibly can, but it sinks me every single time in that narrow passage. What am I doing wrong?