Doing every single attraction and shows plus waterpark would be an uncomfortable stretch and I wouldn’t recommend it.
Doing highlight attractions plus waterpark is doable. If you are ok skipping the gardens, garden-variety coasters, just one show and movie then you shouldn’t have any problems with a short stay at the waterpark. RD pays off big time in Legoland. I would say that even during crowded days most people start arriving about 2 hours after opening. Probably because no one expects for it to be so far!
Arrive very early and RD. Start at the end of the park and work your way toward the entrance. The park is essentially one big corridor. The waterpark is very crowded and nothing to write home about. Don’t expect a mini Volcano Bay because it isn’t.