They have the same warranty as before, but their seals should withstand water. If they detect water damage on a Warranty claim, they'll check the seals to see if they were faulty or compromised.
As long as you keep in mind that the devices aren't waterPROOF and keep to their respective water RESISTANCE levels (i.e. no more than 5 feet of still, sweet water or equivalent pressure for no more than 30 minutes for the latest IP68-rated Samsung phones; salt water is corrosive and most general consumer devices' IP Rating isn't guaranteed for salt water immersion) you should be fine. I take my S7 Edge to the pool with me all the time, but I never go under with it (but it does get pretty wet while using it). I keep reminding snapchatting iTeens that their shiny, new iPhone is not meant to be used underwater (it's got less protection than Samsung's) and even more because the pool in my apartment complex is salt water.
Also remember that water pressure plays a role here, too. It's not the same to have the phone under still water than submitting it to a jet of watter (think jacuzzi or pressurized hose stream). Having your phone in your hand and 'sliding' it underwater while taking a stroke while swimming or just walking around with it also submits it to extra pressure, and the seals might not hold if that exceeds the level stated by the manufacturer or the claimed IP Rating.