Can eight week old ducklings go outside in the day?

You could try it, just ensure they have a place to get out of it, the pool has proper in/out methods? young birds are a bit er... uncoordinated.

 

I agree, definitely let them outside to play but allow them access back inside. At 8 weeks they are fully feathered and pretty much fully grown except for more bulking up they will do over the next month. I let mine out at 6 weeks for a partial day leading up to full days at 8 weeks.