If he "took a bath" the day that his poop was runny/watery, it's possible that he might have had too much water to drink from the bath, that can cause it, or if he ate more fresh fruit or veggies than he normally does, that can cause it too...
***Honestly, it would be a really good idea for you to take him to either a Certified Avian Vet or an Avian Specialist Vet for a Wellness-Exam anyway, if he hasn't had one recently, and since we're at the end of the year it's a good time to do it anyway...All yearly Wellness-Exams need to include sending a Fecal Culture out to a lab for testing and then also having the CAV do microscopy/gram-staining on it immediately in the office during your visit. You should have this done at the very least once a year, along with a complete visual and physical exam, and then also complete Baseline Blood-Work. All parrots should have this done at least once a year by a CAV, due to how well they hide all outward signs of illness and pain...Usually when we as their owners first notice that our birds are or may be sick, they have actually been sick for weeks to months at that point already...
So if I were you, I'd call your CAV and make an appointment for at least a Fecal Culture to be sent out for testing and then also to be Gram-Stained in the office to rule-out that your bird has any Bacterial, Fungal, or Protozoan GI infections, and if he hasn't had his full Wellness Exam then I'd suggest that too...Gastrointestinal infections can linger for months and months, even years without detection or treatment, especially in our pets, and particularly in birds. We don't always look at their droppings, and GI infections tend to ebb and flow depending on what they are eating...So if your bird has a GI Fungal/Yeast infection and his droppings finally become watery to the point that you noticed it, if your bird then goes a day without eating many fresh veggies or fruit and only eats a lot of solid, dry food like pellets, seeds, etc., then this can cause his droppings to normalize or at least "appear normal" the next day, and then you think he's fine, etc. So my point is that he's probably fine, it was probably just related to what he was eating, but with birds you shouldn't chance it, and if he hasn't had a complete Wellness-Exam that included Fecal and Blood testing this year or if it's due again, then this is the time to do it...