Mare Grassia Omishima is a marine bathing facility situated on the coast of Omishima Island, floating in the Seto Inland Sea, in Imabari City, Ehime Prefecture. The name Mare Grassia is Italian for the bounty of the sea, and the facility incorporates the principles of thalassotherapy (marine therapy), combining seawater, seaweed, and the maritime climate to soothe and revitalize body and mind. It is one of the rare complex relaxation facilities of its kind in western Japan. The tranquil expanse of the Seto Inland Sea and its scattered islands stretch directly before the bathhouse, and bathers can enjoy panoramic sea views from the open-air bath and health enhancement rooms.

The most distinctive feature of the facility is its seawater bath, where seawater drawn from the Seto Inland Sea is heated and used for bathing. The minerals and ions absorbed from seawater are believed to promote metabolism, retain body heat, and tone the skin in line with classical thalassotherapy principles. Due to the high salt concentration, soaking sessions in the seawater bath are limited to about five minutes to prevent fatigue. In addition to the signature seawater bath, the facility offers a massage bath, reclining bath, walking bath, event bath, sauna (a 92-93 degree far-infrared gas stove sauna), and a flowing 18-degree cold plunge pool.

It should be noted that this is not a hot spring as defined by Japan''s Onsen Law, but rather a marine bathing facility using heated seawater from the Seto Inland Sea. From the open-air bath, visitors can watch the sunset over the Seto Inland Sea for an unforgettable bathing experience. An on-site restaurant serves cuisine featuring local ingredients. Towels, soap, and shampoo must be brought from home or purchased on site. Admission is 700 yen for adults, 350 yen for children, 560 yen for seniors aged 65 and over, and free for children under 3. Free parking is available for 60 vehicles. The facility is popular as a base for sightseeing and cycling along the Shimanami Kaido.