
History: In the nineteenth century, European sailors wore straw hats with flat crowns and brims. As the design was phased out for military wear, it gained popularity in the late 1800s and early 1900s as a men’s formal summer daytime hat (worn with a suit) particularly at sailing events. The name “boater” was adopted as the hat took off in popularity. 
Around the same time, a similar hat for late Victorian era women and children, known as the “sailor” was widely worn. With a larger brim size than its male ‘boater’ counterpart, sailor hats were trimmed with a dark hatband that extended to ribbon streamers trailing down the back. Women’s sailor hat designs often featured flowers around the base of the crown as well.
Difference Between a Boater and a Sailor: Both hats have a completely flat crown and brim. Traditionally, boater hats are made with stiff straw and are trimmed only with a hatband (in solid or striped grosgrain ribbon). The brim size of a traditional boater is modest- noticeably smaller than a the brim on a sailor hat. Historically, sailor hats have wider brims than a boater and were made of all kinds of weights of straw and felt. 
Difference between a Boater and a Matador: They are also the same basic shape- both hats have a flat crown and brim. A Matador crown is visibly taller and the hat often includes a chin strap. The name is confusing as most Spanish matadors actually wear a traditional Iberian “Montera” hat as part of their bullfighting costume, a hat that looks completely different!
Characteristics: A boater has a perfectly round crown and brim, both of which sit horizontally flat.
Royals Associated with this Hat Style: Princess Beatrix of the Netherlands has long been a boater wearer, as has Queen Elizabeth. Princess Beatrice of York has also embraced boaters in recent years.
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Queen Elizabeth on May 16, 2017; Princess Beatrice on June 16, 2016

Princess Beatrice, June 17, 2017; Princess Tsuguko, August 19, 2013
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Queen Elizabeth, March 24, 2016

Princess Beatrice at Ascot, June 18, 2019; The Duchess of York in July 2021
Boaters are seeing an upswing in popularity and I suspect we’ll see them appear on more royal heads- what do you think of this hat style?
Photos from Getty as indicated and Samir Hussein via Getty; The Asahi Shimbun; Mark Cuthbert/UK Press via Getty; Misan Harriman