Did you go recently? I understand that the Nautilus Explorer (on which I am booked for December!) did a big refurb for 2018 and now have air-conditioning in all the cabins, including individually controlled in the triple and single and centrally-controlled in the rest...
Nope, it was Nov 2017, so sounds like you have an upgrade
you will have a great time. I really enjoyed it. I would certainly take:
Ear Plugs: you sometimes motor between destinations at night and it can be loud, at least if you are in the cabins in the stern ( triple cabin is closest)
Kool Aid drops for the water: I'm a bit precious, but i found the water to be better with flavor in it and you really need to stay hydrated.
Water bottle: to fill up and carry with you on deck
I also had some issues with the 'vanilla glade plugin' in every socket in the salon. I found the smell to be very strong when i was feeling a little seasick and I spent a lot of time on deck because of it...or pulling them out. If you are sensitive to strong smells, you may want to carry vicks with you.
Take lots of alka seltzer/lemon drops/nazal spray/sudafed for the ears and nose/athletic tape to deal with any blisters
Don't overeat any big cooked breakfasts, especially before you dive sites like 'the boiler'
it can be a bit of a swim against the current to get to the wall and on a rough day you can get thrown around a lot. lots of people were 'feeding the fish' through their regulators
You will have an amazing time. It's bucket list palegic diving and the topography of the islands underwater is superb. I took hundreds of photos, just of the rock formations.