NB-IoT certainly doesn't support handover because it's irrelevant for the type of infrequent short messages that it's aimed at. From what I recall (having been in the standards discussions at the beginning) it ought to support roaming because some of the use cases involve tracking assets between countries, but I could be wrong. I suspect the main issue could be coverage of the areas that you want since cellular systems are generally planned for land cover not oceans.
Would the AIS system be a possibility for what you want?