Your suggestion only works with a slight alteration:- Fasai nofio ddim yn well 'i' chi? = wouldn't swimming be better 'for' you?
The construction above is the Welsh equivalent of expressing preference.
Basai hi'n well gyda fi(gen i) = I would prefer etc....
Here's a song with the chorus 'mae'n well gen i........y hogiau go iawn' (I prefer....the real boys)