The use of deep seawater (DSW) in thalassotherapy has begun in Japan. To clarify the health effects of DSW on the human body, we investigated the changes in plasma lactate and pyruvate concentrations, or subjective judgment scores, after bathing at rest in 9 healthy young men. Subjects were immersed for 10 minutes in DSW, surface seawater (SSW), and tap water (TW) heated to 42°C. Plasma samples were collected before bathing, immediately after bathing, and 60 minutes after bathing. The scores were obtained by an oral comprehension test. In the DSW bathing, plasma lactate and pyruvate concentrations showed no significant changes immediately after bathing or 60 minutes after bathing. In contrast, subjects who bathed in SSW showed a significant decrease in lactate concentrations 60 minutes after bathing compared with immediately after bathing. Subjects who bathed in TW showed a significant increase in lactate concentrations immediately after bathing compared with before bathing, and they showed a significant decrease in lactate and pyruvate concentrations 60 minutes after bathing. We found no significant change in the thermal sensation score in the DSW bathing, though significant differences were found between before and immediately after bathing in the SSW and TW groups. Moreover, the score decreased significantly 60 minutes after bathing compared to immediately after bathing in the TW bathing. Higher concentrations of salts contained DSW such as sodium, nitrate-nitrogen, phosphate-phosphorus, and silicate-silicon may have a good influence on human health. Although additional studies are needed to support our findings, DSW is the mildest water to the human body among the three kinds of water, since no significant changes in the items measured were found only in DSW.