Understanding and conserving the ocean's ecosystems is crucial for sustainable development, and therefore, we introduce Sea Guru-I, an environmentally friendly fish-like underwater vehicle inspired by manta rays, capable of sophisticated swimming maneuvers at depths of up to 2,000 meters. This paper presents its comprehensive design, fabrication, and oceanic testing. Leveraging a small-scale model, we elucidate the hydrodynamic advantages of manta ray-inspired features through rolling and flapping tests. Additionally, we provide detailed results from swimming performance tests, showcasing the vehicle's agility and its capability to image free-swimming aquatic organisms. Furthermore, successful sea trials conducted at depths of 2,000 m in open-water marine environments validate the feasibility and robustness of this bio-inspired deep-sea robot. Sea Guru-I represents a significant advancement in the field of underwater exploration, offering promising prospects for future deep-sea research and conservation efforts.