Hey TUF, know more about Cairns but.....
1. Weather will be terrific during the day. Warm but nights can be cool so bring a light jacket. Won't be cold like south though
2. Nautilus Restaurant has a good reputation in Port. If you can make it to Cairns (gee, only an hour's drive), try the Tamarind at the Casino or the Ochre. Real nice places!
3. Most of the big companies are good for snorkelling like Quicksilver, Great Adventures etc with the big pontoons, semi-sub, lunch etc. If you don't want all that, there is a company that does a quick trip out in a smaller fast boat and does snorkelling only. It's cheaper too. Depends what you want. Can't remember the name but you'll find it easy enough when you're there.
Just remember too. The road from Cairns to Port Douglas is one of the most scenic drives in Oz. You'll like it. Have a great hol' !!