In this article we address the 'educationalization' of authentic marine navigation tasks for upper secondary mathematics classrooms and their potential for students' learning. Our approach is based on boundary crossing and as well as on the distinction between realistic tasks (targeting students' familiarization with the marine navigation practices/tools) and authentic tasks (targeting students' inquiry while engaging in genuine events from the marine navigation practices). Through illustrative episodes we highlight how a didactic sequence of realistic and authentic tasks can facilitate 10th grade students' boundary crossings between the practices of school mathematics and authentic marine navigation.