I've dived off Cairns a couple of times taking one of the old pearl luggers a long way out to the reef.
5 divers only, wife staying on board because she couldn't stand getting her hair messed......LOL, plus captain and 2 crew......9 people in total.
I can recommend that.........Could not stand going out with "hundreds" just for a look or a snorkel on some of those huge things that are in Cairns.

Jet skiing I can only suggest Sunshine Coast out to Morton Island, plenty of water around the Gold Coast, Broadwater and ocean, around south Stradbroke to Tipplers for drinks and eats, and north Stradbroke, and even up to Tangalooma on Morton from here......but be aware of fuel situation as there is none at Tangalooma......Amity and Dunwich have it so take drums.

Have a great trip.