If your stammer is purely nerves-related, it can be addressed by confidence-building measures - which the services are well able to provide! It certainly wouldn't seriously affect your chances of selection unless it was quite severe and unlikely to be improved by such methods.
However, before your next set of interviews, it may be worth putting yourself in nerves-inducing situations. If your school/college/university/whatever has a debating society or amateur dramatics club, consider joining. If those don't appeal, try offering yourself to give talks on your ambitions to the local ATC/Scouts/RAFA/Womens'Institute etc. I'm sure you get the idea! You'll learn to relax in stressful situations, which can only help. And the Navy will appreciate the effort you've gone to to alleviate the problem....