If you insist on wearing a burka in a pool, chances are you aren't doing it willingly.
Like a previous poster already mentioned, a "burkini" is not the same thing as a burqa (or niqab for that matter), since it doesn't cover your face. It also isn't that different from the swimwear Western women used to wear around 1900, except that a burkini is of course more aerodynamic in design.
As for the "voluntary" part... it is a bit condescending to always assume that a Muslim women didn't choose to wear a hijab for herself. Speaking from experience here in Berlin, it's not too unusual to see mixed groups of young Muslim women where one half wears a hijab and the other half doesn't. I think it's a bit too easy an assumption to make that all the hijab wearers were forced to, and the hijab non-wearers were not.
A different matter entirely are places like Iran though, where it is mandated by law. These laws are inappropriately restricting freedom... and there's of course the underlying issue of general miscontent with the political system that sparked recent protests there.
Speaking of which, lately it has been a subject of debate in Germany whether it is constitutional to ban women who refuse to cover their breasts from using public pools. If you say that women shouldn't be too uncovered (bare breasts), but also not too covered (through "burkinis") you are creating a very specific, narrow norm everyone should abide to. You know, almost like they do in Iran.
Of course, in a strict sense nobody does anything purely voluntarily. Muslim women who wear a hijab "voluntarily" do so in most cases because they were raised that way or because of the society they grew up in. But that's where we enter a more philosophical spectrum of where the free will begins and where it ends or whether such thing exists at all.
