The icon is a chrysanthemum, the Imperial Seal of Japan.

On ships, they seem to have been made of teak and then painted. Teak is a type of wood that is noted for its ability to withstand weathering and wet environments; it's common in ship construction. The original teak chrysanthemum from the Japanese battleship Mikasa, flagship at the Battle of Tsushima in 1905, was removed from the hull some years ago but is still on display. Even on sunken warships, the chrysanthemum on the bow is still often in good condition.

The chrysanthemum was also used as a symbol on many other Japanese military items at the time; it was stamped into the steel on small arms during the war, but many of these were later scratched out or removed.

The chrysanthemum had no function or practical utility that I am aware of. I can't speak in-depth to the cultural significance of including it (for example, whether it was believed to bring good luck or protect the ship like figureheads on some ships in the age of sail). Perhaps /u/lubyak can provide some additional context.