Which one is correct: "Don't wait for us. We aren't swimming/aren't going to swim today, we're tired.
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question should include more details than have been provided here. Please edit to add the research you have done in your efforts to answer the question, or provide more context. See: Details, Please." – user3169, Catija, Andrew, Rompey
Both are grammatically correct but, in a realistic situation, neither is natural. The reason for this is that, for most people, they have to go somewhere- for example, a beach or a swimming pool, in order to swim. So most English speakers would think of the activity as go/going swimming, or go/going for a swim. In the same way, one might go to the cinema or go out for a meal. The going is an important part of the activity.
Putting that idea into your sentence, you get:
Don't wait for us: we aren't going swimming today.
Don't wait for us: we aren't going to go swimming today.
Don't wait for us: we aren't going for a swim today.
Don't wait for us: we aren't going to go for a swim today.
The don't wait for us part of the sentence lends weight to the idea that the group of people in this particular situation would have to go somewhere to go swimming.
They're both correct but mean two different things. "We aren't swimming today" = we haven't swum and won't swim today. "We aren't going to swim" = we may have swum (or may not have) but, regardless, we won't swim later today.