Coastality, the degree to which a place or community is functionally, spatially, or symbolically oriented toward the ocean, has received limited attention in Ocean Literacy research. This study introduces perceived coastality as a youth-centered construct and develops the Coastality-Gap Index, a spatial indicator measuring the divergence between students’ coastal–inland identity and their actual geographic proximity to the sea. A mixed-methods design was applied to data from 645 students aged 10–17 across 11 schools in five municipalities in Mallorca (Spain). The questionnaire explored emotional, cognitive, and experiential connections to the ocean, including indicators of marine knowledge and spatial self-identification. K-means clustering was used to identify perceptual profiles, which were mapped using GIS to examine their spatial distribution. Five distinct profiles emerged, ranging from students who perceive themselves as coastal and show strong experiential, emotional, and cognitive ties to the ocean, to others who live near the coast yet exhibit limited awareness or connection. The Coastality-Gap Index revealed that 14 of the 29 population centers studied were inland-oriented despite coastal proximity, highlighting educational blind spots not captured by the European Union’s fixed 20 km coastal belt. Together, the perceptual typology and spatial indicator provide a transferable framework for rethinking blue education strategies and designing context-sensitive Ocean Literacy interventions.