There was some changes made to swimming a while back and I'm not aware of if there's a proven formula which manages to work for all cases( the old one wasn't that friendly to work with either).
I will say however, with 50 con, you should be able to swim across about a 20 tile river with a little wiggle room. To provide an extra layer of safety, back off from the river and have your character set to path across it, and right click a waterflask to bring up the flower menu (you need to do this before you start swimming or the menu won't pop up). With this, you are able to drink when you start to get low on stamina (somewhere between 30-50% depending on how risky you want to be with lag).
Please note, whatever you take back from what I've written here, I still hold by suggesting you to either obtain more constitution before swimming, and/or wear a
ferryman ring for absolute safety. I do not take any responsibility for anything that may happen with you if you choose to take my advice, you are the one who makes the choice and are responsible.