For our trip to Japan, we would like to begin with some relaxing coastline. Lots of sun and swimming for us and our 4 year old. Calm waters are prefferable.
I have read here that Okinawa is good... Any particular island? Resort? Beach?

( Someone mentioned that Americans are not favorite guests in Okinawa, unlike the rest of Japan. Is that true?)

Read up on Ibusuki and elsewhere in southern Kyushu for beaches. The Sanyo region around Okayama is another place you might look, since (I think) sanyo means "sunny". I saw your post about strollers and just want to mention that it was obvious that the Japanese adored their children and that domestic tourism via train is popular, so I would imagine that rental strollers would be easy to find.

Thanks. I'll definitelly check it out. Is it "swimmable" temperature-wise?
Water temps/ versus air temperatures?

Oh... I have just realized I did not specify when. End of May and beginning of June...
Are the temperatures high enough for (adult) swimming? I know kids are willing to dip even in an ice water

Oh, you're here now. Japan is not really a destination for a beach vacation, actually. In Ibusuki, there weren't many tourists at the hotel. I didn't go into the water, but got a hot sand bath.