This study proposes a novel hybrid underwater robot platform, Hazardous and Extreme environment RObot for Biomimetic multilocomotion-based Underwater Expedition (HERO-BLUE) that integrates swimming and legged motions. HERO-BLUE is equipped with a multimodal fin comprising numerous passive joints that can act as both a swimming fin and a walking limb. This multimodal fin is integrated with a salamander-like spine and soft undulating fin in a single robot system to enable three representative locomotion modes: swimming, walking, and crawling. This article provides details of the hardware configuration of HERO-BLUE, mathematical models for each locomotion mode, motion planning methodology, and control strategies for enabling hybrid motions. For legged motions, a dynamic simulation study was conducted to analyze the ground reaction force. The results verified the effectiveness of the proposed multimodal fin. In water tank experiments, the swimming capability was validated through thrust tests and swimming trials, and the legged motion capability was quantitatively validated in three environments, including gravel, water flow, and slopes. Furthermore, field trials were conducted in real-life scenarios in open seas, lakes, and stream beds. The experimental results show that HERO-BLUE can successively combine swimming and legged motion capability and increase underwater mobility in various challenging underwater environments.