Background: To date, research investigating workers' engagement in physical activity while working and living in constrictive environments is limited. Purpose: The purpose of this study was to identify the determinants and prevalence of physical activity among mariners and to understand factors associated with exercise while working on the towboat. Methods: The study utilized constructs from the PRECEDE-PROCEED planning model to explore perceived benefits, barriers, and motivation for physical activity (PA) among mariners using a cross-sectional survey and anthropometric measurements. Results: Only 29% of participants met moderate PA guidelines, and 34% met vigorous PA guidelines. Deckhands were most likely to meet moderate (49%) and vigorous (51%) guidelines and were significantly more likely to meet guidelines compared to all other occupations on the boat. Nearly all participants (92.9%) were considered overweight or obese. Nonexercisers were significantly more likely to suggest shift work and weather as barriers to PA. Discussion: Findings suggest that mariners are at increased risk of chronic disease and the constrictive environment of the towboat is obesogenic by promoting physical inactivity. Translation to Health Education Practice: PA changes must be considered because results reinforce the significance impact of outside forces, including split shift work and the built environment, on health behaviors.