loads of people still go to mallorca in september for the sun. i would "hope" it would still be good for sunbathing and sea/pool swimming, but weather being what it is, there's no guarantee and at that time of year, less so.
More than warm enough for both yes, that’s not the problem, thunderstorms are going to be your problem. They don’t tend to last very long, i.e. rarely will you get bad weather days on end, usually just a couple of hours or overnight but they can be pretty spectacular. Next day you can be on the beach as if nothing happened.
You might kop a storm but the sea is nice to bathe in that time of year .