Gidday Midwest,
You are not going to believe this, it has been 10 knots and sunny skies here in Cairns for the last two weeks and every single dive boat ( 60 of them ) have been travelling half full. It is just fantastic weather.
Now to answer your questions.... weather patterns for Cairns last 2 years:
Feb 400mm rain
March 800 mm rain
April 1600mm rain
May very windy and topsy turvey, doesnt matter for divers however right? we stay under the water.
Availability on all boats whether day tours ( 5 hrs up to 3 dives), liveaboards 2/1 night..3/2 night, or extended is always fine with standby's available for extended Cod Hole tours.
Having advised for many years as the oldest independant advisory service here in Cairns, my advice is to wait until you get to Sydney or even Cairns to try and get the very best day or days for your GBR experience.
Budget: day tour snorkel approx $100 to $150 for day. Add $100 for 3 dives, guided full gear.
Liveaboard: $300 2 day / 1 night 5 day 1 night dive full gear
Extended Liveaboard starting $1000 approx 10 dives full gear.
New boats upgrades over the last 3 months are Tusa, Seastar, Reef Experience in sea trials now.
Trusty one day reliables with quality reef.... Silversonic, swift, calypso.
Liveaboards to search are Pro Dive ( 3 day) , Deep Sea , Reef Encounter.
Extended in my order of brilliance Mike Ball, Spirit of freedom, Nimrod then Taka. Undersea is to long for your visit.
Now your overloaded however May is a long way away.
Cheers Andy