I've been to Hamilton Island quite a lot over the years, and TBH, although they offer diving, it's not a really a great diving destination. You really want to be further north for the good diving. I stayed on Hammo in April, and did the Reefworld trip - mainly for the experience and snorkeling for the kids, but I packed my regs and did a dive there. My partner also did a try dive (she'd done a discover scuba in Melbourne a couple of weeks prior, but is not yet certed.
She absolutely loved it! I was a bit meh. There were a lot of fish around, and a couple of large resident groupers, but there was also a fair amount of coral bleaching.
On the trip out there, they'll do a massive spruik on the try dive. I was in the "experienced" group, which included 3 other cert'd divers, all who hadn't dived for a couple of years. It was very much a follow-the-leader dive, and none of the DMs came across as particularly experienced.
There were about 30 people doing discover scuba.
If your diving has been in Melbourne, there is different stuff to see, but not necessarily more interesting. IMO, even the likes of Blairgowrie, Rye and Flinders piers keep me more interested.
But having said all that, with less than 25 dives under your belt, I'd say go for it - just for the experience.
My dive was max depth 14m, with most of it spent around the 7m mark. It's possible to go deeper, but most of the interesting stuff is in the 5-15m range.