I’m going to have to disagree with her surviving for months at sea. It’s not so much that she couldn’t swim down to where the sun couldn’t reach her; it’s that eventually she’d need to sleep. A vampire cannot go months without sleep, whether they sleep like a mortal or they just pass out at sunrise. She might manage a night or so without succumbing, but she would sleep eventually and then she’d just float back up to the surface and be killed by the sun. If you want to adjust and only have her lost at sea for one or two days, not two years, I would find that more acceptable.
Also, please read through it again, I noticed a few little typos. For instance, “there fortune” should be “their fortune”, and “there planning” should be “their planning”.