The Seaside Resort is a standard tile improvement in Civilization VI. It can only be built on coastal Grassland, Plains, and Desert tiles (no Hills) with a minimum Appeal rating of Breathtaking. In Gathering Storm, it can also be built on any coastal tile (including Snow and Snow Hills) with Volcanic Soil that has Breathtaking Appeal.
For the player pursuing a Cultural Victory, Seaside Resorts provide an alternative source of Tourism which may help immensely! Unfortunately, their placement requirements are quite specific (like the National Park), so you should plan in advance. Found cities close to or on the Coast, in lowlands, and avoid building stuff which diminishes Appeal in the tiles immediately next to water. The Eiffel Tower Wonder will help you immensely with placement opportunities, because it will increase the Appeal of all land in your empire.
Seaside resorts have been around since the Roman Empire, the first where folk – rich folk – had the time to spend lying about; the Roman town of Baiae on the Gulf of Naples, the island of Capri and the doomed town of Pompeii were all favored by the elite of the empire. But it wasn’t until the mid-18th Century that the wealthy took to the waters at seaside resorts in droves as part of their conspicuous consumption; the first to cater to the indolent rich was Scarborough in 1720 AD. In 1793 Heiligendamm opened, bringing Europe’s aristocracy to the shores of the Baltic. With the coming of the Industrial Revolution, the nouveau riche joined them in splashing about in warm waters. Then, as working hours declined and disposable income increased, even the working classes could join them at Brighton, Nice and Coney Island … forcing the filthy rich to go to places like Monte Carlo and Acapulco for their sun and fun.