It's actually harder to sink a ship than it looks. I've done this... I was a Coastie in a former life, many years ago. I got to personally put some holes in and sink a few things out there. Derelict vessels are not uncommon, and we'd routinely sink them to keep them from becoming hazards to navigation. That-- and it was just plain fun.
Ships are designed to float and they have lots of spaces that trap air... From watertight working compartments to various voids and tanks. A little bit of air has enough bouyancy to support an enormous amount of ship. Sinking a ship that large will mean poking lots of holes in lots of places. They're probably in no hurry either. May as well drag it out as long as possible and use it as a great gunnery exercise.
My ship had a 3" deck gun (big, big fun!) and a couple of 50's. It only took the tiniest excuse to tune up the guns and play a bit. "Gee, cap'n... That there looks like a hazard to navigation to me... What do you think...?". :-)
*snort* Boys and their toys!
“It’s actually harder to sink a ship than it looks.”
So perhaps we could use this as the new motto for the Coast Guard. “This Job Is Harder Than It Looks - Go Coast Guard!”