Paddleboards like the one in this photo had been invented by Tom Blake in about 1929, then manufactured in the 1930s.
Hollow inside for more buoyancy and less weight, they inevitably leaked to some degree, so a metal plug was also included to drain the inside once the board was back on land.
Although there’s no date on this amateur photo, it is likely to have been taken during World War IIor soon afterwards.
The hairstyles and bathing suits on the women in the background on the left are one clue. In addition, groups of men at Waikiki like this snapshot shows are usually wartime military buddies on their time off.