The deep sea fishing season in Thailand (3-4 days trip out to Andaman Sea) starts October - May. Now it is a monsoon very few boats venture out very far. It normally costs around £150-£200 for a trip to Similan Islands. To really target the big game fish such as big tuna, big grouper, Snapper, you need to fish the Burmese Indian Ocean (Burma Bank). This typical trip will cost you around £400 -£450 for 5-6 day trip. It sounds expensive but fishing in Thailand will include 3 meals most days plus snacks and soft drinks although people usually bring their own beer and whiskey
If you stay in Krabi, the nearest good fishing port will be Tub Lamu, Pang- nga province north of Krabi. I am sure there are good boats in nearby Phuket but Thai people including me tend to go out from this har bour or the one further up north at Ranong province.
Here are popular boats with good skippers:
Polaris - sails form Ranong, Tel: (09 ) - 289-9159 http://www.siamfishing.com/content/view.ph...03&cat=trip
Tai Leu - sails from Tub Lamu, Pang nga, Tel: (076) 443-068 http://www.siamfishing.com/content/view.ph...06&cat=trip
Fishing from shore around Krabi (I have no experiance here) I guess people then to use strips of squid, or small prawns to target palm-size reef, rock fish which there are plenty around resorts.
If you go in a few weeks time, one-day in shore fishing should be available on most resorts but I have no experience to suggest any particular boats.
Hope this helps