I agree, we did exactly this a few weeks ago, stayed at the Hard Rock for 1 night, checked in at around 7, got our fast passes sorted and then straight to the park. If you're family are into Harry Potter it's definitely worth getting tickets for the 2 universal parks, which will also enable you to ride the Hogwarts Express.
It's interesting to hear that you enjoyed Volcano Bay Bozza, we read such bad reviews that we gave it a miss and stuck to Blizzard Beach and Typhoon Lagoon for our water based fun.
Agreed - reviews have been mixed, primarily due to it getting full to capacity, sometimes not long after 9, primarily due to on-site guests having early entry.
As a family we're not great at getting into parks early so we didn't take advantage of the early entry, even though we had 4 mornings to give it a go.
Instead we headed over one day around 4ish and stayed there until closing at 9. We got on a fair bit of stuff in that time, and the tapu tapu things work well.