"Maker's breath, either do it or don't. Stop berating me with these questions!" Isabela wasn't actually mad, she was just tired of this subject. She wanted people to just leave her alone about it.
There was also the fact that Isabela seemingly wanted Lara more than a ship and that fact scared her to death. She'd betrayed Hawke for the promise of a ship. And yet she wanted Lara more than a ship. What kind of pirate was she if that was the case?