So dwarves weigh between 43 and 122 kg, an average of 82.5 kg.
We already know that dwarves have an average body size of 60,000 cubic centimetres.
That puts their average density at 1.375 g/cm^3. Water has a density of 1 g/cm^3.
No wonder dwarves can't swim.
In calm water you could keep that afloat with sufficient effort but you're assuming that these weights and volumes scale proportionally. Outliers could skew either value so what we really need to know are the mean values for these, given a sufficiently large sample.
actually if you take a human that weighs 150 lbs. put a 60 lbs. weight belt on him without fins and such he could swim if he was really good at it, not for too long though, but he could jump off the bottom pretty good. with us humans drowning has more to do with fear and panic than buoyancy, though +60 lbs. in water is a lot to overcome. i weigh 120 which would be 48 lbs. at 40%, and i am pretty good at swimming. the rate at which i would burn up my oxygen opposed to how quickly i could get it with the back and forth of bringing your head out of the water would mean that i could keep it up for more than 1:30 minutes to 2:00 minutes, which seems like the swimming skill is pretty accurate then. now if you take into account jumping off the bottom to get air when at 15 feet or so and staying fairly relaxed then i could survive a lot longer than 2:00 minutes, and get out if i wanted as well.
in the end the number gives credit to the swimming skill, so it might very well be accurate for dwarves.
oh yeah, rough water doesn't sink a self contained vessel any faster than calm water, so you might drown faster in rough water (breathing waves), but you will still have the same buoyancy and thrust characteristics.