The Spaniard warships approaching Nassau in the episode's last scene display at the top of their masts the red and golden-yellow flag -commonly known as "la rojigualda"- that modern audiences are expected to be able to identify immediately with the Spanish empire. However, by the time of the events depicted in the series, specified to be happening circa 1715, that flag didn't exist yet - and it wouldn't for another 70 years, to be exact. King Carlos the 3rd of Spain did chose this particular design amongst a selection of 12 final ones which were the product of a public tendering process held by the Crown's Minister of the Navy, Antonio Valdés y Fernández Bazán, and the final decision was made official on May 28th, 1785.
The flags those ships would've been sporting actually would've been the red Cross of Burgundy over a white background as standard -that part they got right at least- and the banners at the top of the masts would've been a representation of the House of the Bourbons' coat of arms, most probably displayed against another Cross of Burgundy over a white or yellow background. But never a rojigualda because there's no way about it: in the year 1715, it simply didn't exist yet.
The flags those ships would've been sporting actually would've been the red Cross of Burgundy over a white background as standard -that part they got right at least- and the banners at the top of the masts would've been a representation of the House of the Bourbons' coat of arms, most probably displayed against another Cross of Burgundy over a white or yellow background. But never a rojigualda because there's no way about it: in the year 1715, it simply didn't exist yet.