ICW depth...perhaps others are more qualified to answer whether 6'4" is suitable. I read in the past year, in magazines and newspapers ,that certain parts of the ICW in the Carolinas are shoaling up, and in some spots, finding 5' is difficult...one report had charter fishing boats running miles out of their way to get to the ocean because of shoaling. If you are going to be in NC waters much (outside the ICW), then even 5 ft. draft is going to limit some of your movements...at 4'11", I plowed the marina fairway and creek to get to open water two weeks ago, because a west wind had dropped the normal water level about 1 ft. below normal. (Fortunately the bottom on the creek is soft).
While I agree with Stan regarding preferring the NC331 over a C320 for storms in the north Atlantic (if for no other reason, it'll be warmer and dry with the inside steering station over the C320 open cockpit), the people who determine what makes a Cat A, B, C rating probably have it right. Sliding doors, unless heavily constructed to guard against it, are more likely to get ripped off than smaller hatches with drop boards, and then, no matter how well the boat is built, it is subject to flooding in a storm with breaking waves.