Can my watercress be saved?

Maybe you could cut down the top a bit so that the struggling plants have less to support. Stand it in another tray of water. If all else fails buy some healthy looking watercress from the greengrocer pick a few good stems and stand in a jar of water for a few days they will root quite easily then you can plant new. Have you tried land cress? Peppery like watercress but easier to grow, and once established seeds itself readily.

Also of there's any healthy-looking bits at the ends of the shoots -put them in as cuttings.

Hi TussieMussie, I took your advice and already the plant is starting to look better. The yellow is turning green again and it's picked itself up. Thanks
Hi Hoya, how much do I cut off and how what do I do with them to keep them alive? As you can tell, I'm new to this :)