We sailed on Navigator in September Live Blog, with my kids who were 6 and 9 at the time. We did not purchase water park tickets at the time. We did consider them, but chose not to because both kids weren't tall enough to do all of the slides. We told them that we couldn't do the whole water park because they weren't tall enough and they had plenty of fun in the free kids area. My oldest made friends there and did not complain one bit that she couldn't do the other stuff. Some of that is they didn't know what they were missing etc. They do know that eventually we will make it back there and by then they can do the whole park. Our 6 year old really liked the free area as well. Royal Caribbean has set a 48" height requirement for all the slides in the thrill park except for the splash speedway slides (42"). For the wave pool and adventure pool if he isnt 48" he must wear a life vest, so this must be taken into consideration so if your 6y/o is that tall and enjoys those, then by all means its probably worth it. If he isn't consider skipping the paid section and let him have free reign in the free area, he will still have plenty to do there, and may feel like more to do since there are no restrictions.
Just my 2 cents. Have a great cruise.
