I don't know about the seasons of the Galapagos but the Andaman sea side of Thailand it is the wet season/low season and is a real punt as to whether or not you'll get reasonable viz.
Also you'll be restricted to the phi phi area as the sea conditions off Khao Lak are terrible- most boats move down to work Phi Phi/Koh Doc Mai which are the best dive site close to the mainland making the live aboard trips less worth while- you could dive the same spots, the same number of dives, sleep in a nice hotel, eat better food for the same money albeit you travel around abit more, but not much. It cost around 5000baht/day on a live aboard, a day trip is 3000 baht and if the weather/conditions are bad the next day, you can pick and choose unlike on a live aboard. In any case you will get better value NOT booking ahead as franchisees reduce their margin to get any sale.
My wife is Thai and used to work in the alcohol wholesale management side of tourism and real estate, we have worked out what is the best value there. pricing things out thoroughly and looking at the service and quality side also. For me it is cheaper to dive Thailand on holidays then going just up the road to Cairns,Townsville or port Douglass- even if the diving is better here in Australia, which it is!!!!!
Prices here in Australia are starting to fall too, that and a few of the inefficient operators going out of biz.
I think the discounts reflect consumer fall off; both annually with the end of the fin. year and the prevailing economic global environment(ie;spain unemployment 57%).
Public services are costing way to much in developed countries, the bloated levels of staffing and overtly high senior administrative renumberation, pay and conditions above its counter parts in the private sector and the inability/unwillingness to retrench/layoff public servants are spiraling down thru to the revenue base of all governments- the delays in reduction of spending and the low productivity of the public sector is going to make the ride even harder for those directly attached to that end of the economy- which likes to see it self as impervious and "above" the private side.
The private side having already taken its hit from the seemly unfettered fraudulence of the banking sector.
The eventual raising of taxes means less disposable income to go diving with........so if your got the bucks, lack of demand is making some value to be had. Lots of operators have "specials" available now.