Triploid or seedless watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] cultivars often have erratic germination and low seedling vigor. The morphology of the seedcoat on two triploid cultivars—Tri X 313 and Tri X Sunrise—was examined by scanning electron microscopy (SEM) to identify structural differences compared to diploid seeds. Triploid seeds incubated with oxygen-enhanced treatments that included nicking, 1% hydrogen peroxide (H2O2), and 40% oxygen were investigated at low and high medium moisture levels. Triploid seed has a thicker seedcoat with a dense endotesta layer and a larger and highly variable air space surrounding the embryonic axis as compared with diploid seed. All cultivars rapidly imbibed water (≈50% of the original weight) during the first hour of imbibition, with a faster increase for triploids than for diploids. High moisture affected germination to a lesser extent in diploid than triploid seeds. Triploid germination under low medium moisture ranged from 96% to 76%, but was severely reduced to <27% under high medium moisture. Triploid seed germination was significantly improved at high moisture by H2O2 and by 40% oxygen. Triploid watermelon seed is very sensitive to submerged conditions, possibly due to a combination of physiological and morphological defects. The rapid imbibition and excess water collected in the seedcoat and air space surrounding the embryo, could reduce oxygen diffusion and impair metabolic pathways leading to normal germination and seedling development.