I don't know why but the idea of a peasant in the dark ages taking a day off and hitting the beaches of say, Barcelona, and building sand castles while there is a literal castle to defend against Moors seems silly to me but at the same time it seems like human nature to want to relax and get some sunshine or play in the ocean.
More can be written, but you might like to start with "Did ancient people go to the beach?" , in which /u/DarthPositus gives a direct answer for Rome (and /u/bryanoftexas even mentions sand castles!). Also, /u/Searocksandtrees pointed to older discussions -- specifically the first two, though they are rather old and single replies are not as thorough as today. The ones labeled "most-definitely-not-ancient-times" are indeed not ancient and not really that old.
This is not to discourage discussion. More questions, data, and debate are welcome.