I will have to spend some time in Seoul for my job, in late August, early September and my wife and my 2 kids will join me.
Now I would like to know if it is possible to make a day-trip to a beach from Seoul. I know that this is unclear, too broad, etc. The following specifications (hopefully) narrow down the scope:
Reachable by public transport (metro, bus, train) in maximum 2h30 from Seoul, knowing that we plan to stay in Sinchon, not too far from the homonymous station.
It should be possible to swim in the sea, i.e. sufficiently safe and clean water.
There should be some basic infrastructure nearby, such as toilets and possibly even showers.
Extra facilities or animations for kids would be appreciated, but are not required.