A tortoise can turn itself over but it can take a lot of work. The shape, size and health condition of a tortoise affects its ability to turn itself over. The rounder the shell, the easier it is for a tortoise to turn itself over but if a tortoise is large or sick, it will be more difficult.
Tortoises cannot swim. At most, they can float and drift, and if they’re lucky they’ll bump into land. Some species of tortoises can swim poorly, but most will simply sink and drown.