Speaking from experience, waders full of water is NOT fun.
I ride 20+ miles on my bike regularly but I was very concerned REAL quick the few times I've actually swam around with some water in my waders. I got tired rapidly. Staying calm might be the best thing you can do; if you panic you're likely to kick harder and tire even faster. Even without water in your waders, actually swimming is tough.
Splashtops (Aquaskinz or otherwise) slowly burp out the air trapped around your shoulders. True drytops (latex gaskets) keep air in longer, but the seal around your waist will slowly let water up and into your waders. I've intentionally experimented with various splashtops and drytops. Depending on your fit, it could happen quickly or you might have several minutes. Either way, once that air is gone, things are going to get exciting if you're still a ways away from getting back on land.
A lot of people view fishing in a wetsuit as "extreme" but really it should be viewed as a "safer" way to fish. You're going to float a bit AND you've got time in cold water if you do get sucked out. You also never have to worry about leaks in your waders.
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