Due to its rich marine biological resources, Taiwan is a place worthy of developing marine ecotourism. This study explores the marine ecotourism intentions and marine ecotourism behaviors of tourists during the COVID-19 pandemic by using the unified theory of acceptance and use of technology (UTAUT) method. Under the UTAUT framework, this study considered the factor of environmental attitudes and issued questionnaires in four research areas, namely, Yilan Wushi Fishing Harbor, Port of Hualien, Hualien Stone Stairs Platform, and Taitung Chengfong Fishing Port. In total, 431 effective research samples were collected. First, this study verified the validity and reliability of the dimensions through confirmatory factor analysis. According to the results of structural equation modeling (SEM) analysis, performance expectancy, effort expectancy, social influence, and environmental attitude all have an influence on tourists’ marine ecotourism intentions, while facilitating conditions had no influences on tourists’ marine ecotourism intentions. Tourists’ marine ecotourism intentions further influenced their marine ecotourism behaviors. In addition, this study evaluated the mediating effect of marine ecotourism intentions by nested-model analysis. Finally, this study provided substantive policy recommendations as a reference for tourism management units and local governments.