"Marcus in piscina natat."

Translation: Marcus swims in the pond.

[jaiirapetjan]

Now we know that Marcus is a fish.

[PERCE_NEIGE]

Piscina is a swimming pool/a pond/a fish pond/a water tank/a public bath/a private bath/a place to wash your clothes;

= all kind of bodies of water

Swimming in a pond is quite possible. If you make an internet search, you'll see than even nowadays, a lot of people in the world do that.
I'm pretty sure than Romans did the same in natural ponds. Why couldn't they? We tend to project a lot of modern cultural reluctances on them.

[Thomas50510]

But swimming pool is not accepted here

[Silicon54]

...or a target of the Corleone Mafia.

[BeefTestos4]

Now I know the ancient Roman gangster doublespeak. Thanks, Duo.

[JMSHG]

Perce neige,
Even if you are right, it was not a reason for refusing the translation of piscina : swimming-pool !

[DeadAccount.]

Dude, that's for fish, get out of there.

[Martin960556]

Pronunciation "Natat" is off

[Kyihsin]

I agree with the comments that "swimming pool" should be accepted (which is not to say it's the ONLY translation that should be accepted!)

[McGonnagle]

with macron, Marcus in piscīnā natat.