"Now, let's swim."
Not all words will be translated literally, but rather it is the same expression as "well" or "now" when it's an invitation. To add a bit more context (or at least from how I perceived this) imagine someone is sad and their friend is comforting them, and their friend knows they like to swim. "Now/well, let's go swimming"
Why isnt いま/今 accepted for 'now'? Thats literally what it means. I understand that its meant to be more of an expression, but I think either いま should be accepted too or maybe change the English phrase to 'well, lets go swimming' or something.
It's not "now" in a literal sense; it's more colloquial. さあ adds a bit more informality to it, like an enthusiastic "Let's". (I'm sorry if it isn't clear; it's hard to explain.)
I already saw so many anime with people shouting this construction. Always wondering "how can i learn that?". T~T now that duolingo give us oportunity, i am so happy.
Stop complaining, it's swimming time