They should be almost fully feathered at 6 weeks. What temps do you have them at now? They should be at about 65-70 at this age. If that is your daytime temperatures they should be fine. If it's lower you shouldn't put them outside without supplemental heat if they aren't feathered. Short periods is fine but not all day and definitely not if they aren't used to it.
They can go for a swim now for short periods too, like 10-15 minutes to start. Increase the time as they grow stronger. Just make sure you towel them off when done and put them back in the brooder to warm up and dry off. Swim time will help stimulate feather growth too.