there are some very tricky new AP rounds avail in .50 cal. the swiss even trialled sabots for them (pretty damn small, but hey, they were showing whats possible).
a .50 cal will go through a hull, but you need to get it right and hope that its got enough energy to reach the drive train and disrupt things.
I was involved in some kinetic tests using .50 cals about 6 years ago and they could cause some serious damage. the idea of warning shots and escalating those shots with an increase in calibre is designed to show resolve and intent in an orderly fashion.
the problem is when they call your bluff and don't stop
Thats why we fire on the bridge & then board. We really don't want to sink them because if you impound the ship the law allows for forfeiture to the Crown plus sinking of FV causes all sorts of environmental problems but worst of all the paper work involved - it would be horrendous