I have used a wrap around leg band for ducks that are numbered but the numbers come off. The leg band sometimes comes off too but if only one comes off at a time it is easy to figure out who it belongs to since only one duck will be missing a band. There is a spiral type I won't use for ducks because I had a problem with it cutting into a duck's leg with the sharp point of the spiral ends. Zip ties probably work best for growing ducklings. I make the mistake of keeping them too loose so they fall off but I would rather have to put them back on again than get them too tight.
I would love it if they made legbands from a rubber ring like the ones used to band lamb's tails and male dangling parts because there is a tool that stretches them but then they hold their shape when they are released. The round rubbber ring would be comfortable for a swimmer, I think, and it would stay on because it would not stretch without the use of the stretching tool. Maybe there is a rubber grommet that would work if it was just the right size.