When you rent a holiday cottage with a private hot tub, it would not be essential to wear swimwear as long as the hot tub is not overlooked by passers-by. The cottage would have to be very secluded for this to be possible.
Obviously, standards of decency have to be maintained in any public area or it could be an offence.
However, if completed shielded from view, anyone renting a holiday cottage that has a private hot tub is welcome to dress as they wish. Some owners take a view on lettings to naturists but this is not normally a problem.