One of Hokkaidou's most spectacular ocean-side baths, Mizunashi Kaihin Onsen is simple and pure, and only usable when the tides are low: each cycle the high tide covers the baths, so you have to be there at the right moment to bathe. Timed right, it's a wonderful soak, though there were some small crustaceans that were biting at the time the author took a dip. There's a small changing area, but any bathing is entirely in the open, so this isn't a good option for the bashful. You are, apparently, allowed to wear a yukata or swimsuit if you choose. The baths themselves vary in temperature based on the tides, outdoor temp, and other variables, but will be warm to hot. The rocks can be extremely slippery, so be careful.
Getting here takes a little work, and is far easier if you have a car, though apparently buses will get you fairly close. Tourists will likely be staying in Hakodate, where the tourist info booths have nice tide charts to make the timing easier. If you can't get a hold of one, just check the web for the tides -- you'll have two times daily where the bath is usable. If you're really lucky, come at night when the moon is full. Amazing!
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