In the past it was thought that swimming in the sea was bad for you.
In the past it was thought that swimming in the sea was bad for you / In the past swimming in the sea was thought to be bad for you.
People know that she is a good swimmer.
She is known to be a good swimmer. / It is known that she is a good swimmer.
They said Francis was in hospital last week.
Francis is said to have been in hospital last week.