The exploration of underwater environments presents unique challenges that conventional propeller-based underwater robots struggle to overcome due to their inherent limitations in maneuverability and environmental disturbance. This has lead to the interest in bioinspired robotic systems capable of multimodal locomotion, inspired by the versatile movements of marine animals that excel in both swimming and benthic locomotion. This article introduces the design framework of a novel bioinspired underwater vehicle, RoboIguana, which draws inspiration from the marine iguana's ability to perform both undulatory swimming and benthic legged locomotion. We present in detail the design of different subsystems of the robot, including legged and swimming locomotion, vision, electronics, and chassis. The integration of these subsystems aims to enable RoboIguana to navigate complex underwater terrains effectively, offering an innovative tool for marine exploration and research.