Is "vijfentwintig schildpadden zijn aan het zwemmen in het water" wrong?
No, that's correct too! :)
Strange... I just wrote exactly that and it was wrong. I even showed it to my partner (native Dutch speaker), who say I had no typos nor other mistakes puzzled
Why is having "er" after zwemmen wrong here?
Why is it incorrect to answer vijfentwintig schildpadden zwemmen in de water?
Because it should have been 'het water' (not 'de water'), since 'water' is a neuter gender word.