Perhaps a native speaker can confirm, but I think "dylet ti ddim" is simply "you shouldn't" whereas "dylet ti beidio" is a stronger suggestion that you shouldn't do something whilst avoiding giving a direct command not to. Literally it means, "you should cease to swim there," but that's not very good English.
If that's the case then the answer given is incorrect, as you should stop swimming, and you shouldn't swim are different statements
Yes, "desist from" was the verb I couldn't think of when I wrote my initial reply.
So, I looked it up online, and it seems like 'yno' is 'there' in north Wales and 'yna' is 'there' in south Wales? Is that correct?
No, not really. yno means 'there'. yna is often used for 'there', and it also means 'then' (as in next in a sequence of actions). In the various dialects there are a variety of expressions used for 'here', 'right here', 'there', 'over there' and so on.