My 10 year old son and I visited Adventure Island on Sunday, July 8. We had a fantastic time. We got there just before they opened, got a locker and some chairs, and were good to go for the day. We caught the big rides first before the lines got too terrible, and then spent the rest of the day cliff jumping, fluming, hydrotubing, and floating in the lazy river. Adventure Island has done a great job of offering areas for little kids and big kids to play. The toddler section with all of the water fountains is great, as is the big kid area with lots of different colored slides, some of which go faster than the big flumes!
There were plenty of people there, but it's summer, so if you don't expect to deal with crowds in a water park during the summer, you might should consider a different type of trip. The food was great, no pricier than any other theme park (and cheaper than Universal), and it tasted GOOD. Especially the barbecued pulled chicken sandwich - just awesome.
I do recommend water shoes to walk around in, as there is of course a lot of wet concrete to walk on. You can wear the water shoes on all the rides, so it's no problem. I really like how there are multiple rock water falls that flow into streams that run throughout the park to walk in, sit in, play in, whatever you like.
Water Country USA in VA is still my favorite water park, but Adventure Island is really great as well, and I highly recommend it.