Lots of times, but just ordinary swimming in the ocean can be tiring and irritating to my eyes. I way prefer scuba diving in the ocean and doing just plain old swimming in a fresh water pool.
According to Webster's Dictionary "swum" is the past participle of "swim". Ergo, "Today I swim, yesterday I swam, and often I have swum in the ocean". Participles can be annoying little buggers.
North Atlantic, North Pacific, Cantabric mand Mediterrarnean Sea, South and East China Sea, but many places to try missing...
You can find beaches with waves and beaches with gentle movement.