I have always been a profuse sweater and during the summer months I take in ~ 60 oz of water during a 90 minute longer session. Even in the winter with water temps in the 77ish range I sweat like a beast. For an equivalent level of exertion out of water I don't need nearly as much water.
Any of my fellow smart peeps here know of any science behind why? I don't think I'm the Lone Ranger here as I know a lot of folks who experience the same thing, but one thing I'm continually shocked at is how few people have any drinking water with them at all for a swim!