salty ocean water makes you float and should increase swimming distance. even double seems viable given the size of oceans. i believe this is in line with developer wishes of having different terrains give different effects. it would be akin to swamps draining your stamina. it also serves as a small bandaid for people dying all over the place by boats breaking.
You could double the swim distance, but at what I suspect is the average CON, it wouldn't matter much because nobody could swim far enough to make it back from deeper ocean.
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