They have quite some buoyancy indeed, a little bit more tiring because of the thickness yes, but not an issue IMO.
You are talking about a swim without your board I reckon - why ?
It would be much better to swim on your board, and here even a normal membrane drysuit works fine as you can swim/paddle when on your board.
But they are, as I've heard, not that good for longer swims in the water ?
Dont know if true or just a rumor, anyone got experience ?
Here most use these membrane drysuits, much lighter and faster to change and warmer - but when IN the water, the neoprene drysuits are better in each and every way I would say
Eventhough I can not see any difference between a neoprene drysuit and a really good thick neoprene wintersuit - they are both a hassle when changing, if freezing
But warm when in the water, and the best for swimming.