While the popular idea of Izu to most people may constitute onsen, Commodore Perry and Shirahama Beach, there’s also some top class diving to be had in these parts.
Ose (大瀬海水浴場) is a small patch of beach in an attractive natural bay. It’s alongside a small peninsula which forms a nice little 20 minute jaunt on foot and houses a shrine and a pond. While Ose Beach sports a campsite and a bunch of reasonable guest house accommodation, it’s a place far from the big roads and main attractions. Nonetheless the beach can get crowded in summer.
The big draw here is the front row seats for Mount Fuji, which is best spotted on clear days, early in the morning or at sunset whilst laying horizontal and paddling away the time. Lacking in big swells the sea is pretty good for swimming, though with such a flat bay one could argue it’s not the most exciting place for a dip.
Out of summer season, sunbathers and swimmers are outnumbered by divers and the beach is literally covered in diving gear too. The diving school operators don’t seem to mind swimmers, in fact they will be bemused more than anything else if you swim out of season.
To the north-west of the beach, up along the little peninsula there are some good spots for jumping off quay walls into the calm, transparent waters, though you might want to make sure you are not being watched when you try it as it’s surely deemed irresponsible and forbidden by the hobbying fishermen on their own Izu trips.