For me it depends where I paddle and the conditions. On the local river I usually dress for a brief immersion only. I would be cold in the water for even the accidental swim and remount. I would be very cold if I need to swim to shore on my own, but I estimate I will still be in good shape to do it. In winter on a sunny day this means dry suit with a thin or medium layer under, in Spring - I might go for dry pants with a short sleeve dry top, summer - just the PFD and a light colored rash guard (well, shorts too
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For open water I dress much more conservatively with heavy layers under my dry suit or otherwise that would allow me to swim for a long time if I must (couple of hours at least, as that's how far the shore would be for me in the worst case scenario that I don't get picked-up by one of the plentiful boats quickly. I also carry a VHF marine radio on my PFD there, a paddle float on the ski, ski to leg leash, whistle, head gear (NRS Mystery or even a full hood if colder), gloves, water shoes with neo socks over the latex booties on the dry suit, etc. I even sometimes carry a canoe paddle as a spare.
And I I have someone who knows I'm out and needs to alert the coast guard if I'm not out within the expected time.
A good strategy is to paddle with others. But I usually go out alone, so I tend to stick to places where I can swim out of and take a bit more stuff with me, to be self reliant.
I tend to take intentional swims from time to time to test my gear and to see how I feel after a few hundred yards swimming in the water or doing self rescue practice over an extended period of time (not necessarily in the ski, but the experience carries over). A quick dip and remount are very different from an extended swim of even 10-15 minutes in cold conditions... I must admit I have not tried to soak for hours in cold water, so whether my layers are enough for a full body immersion over a long time is anyone's guess, but I figure if I have on my upper body similar layers that keep my bum and feet warm in the ski with water washing over all the time, I should be good for a couple of hours swimming too...