I am checking this website (only Japanese) out everyday, and the water temperature is getting warmer and warmer than 10 days ago.
There is a warm ocean current "Kuroshio" that flows northeastwardly off the coast of Japan into the northern Pacific ocean.
The water temperature is changing depending on this current, and this current is getting closer to Izu peninsula in these days.
That's why the water temperature is very warm here in Shimoda.
If you come to Shimoda during this Golden Week you should bring a swim wear.
The air temperature is also getting warmer day after day!!