Like in real life, if you over water plants they die.  Sims will always have the action showing even if they can over water them.  Sims are silly creatures after all :tongue:
Like suggested either limit your sims to water them no more than once a day 
or hire a gardener (by using the service option on your sim phones) that will never over water them 
or use twojeffs mod like suggested 
or you could also use the "daily gardener" mod found from 
this post  if you prefer.