I spent a few years of my life living in Cuba in my childhood and went back many times.
Assuming a change to more moderate form of socialism where foreigners are allowed to own property, Cuba would be the one country I would give living up in Canada up for - unless we could somehow annex Cuba.
Anyway...to your question:
Make friends with the cook in the resort. On the first day, give him a 5 CUC tip with clear instructions what it is for, and he will secure some prawns and raw squid for you every day. It will also get you some extra goodies that may not be on the menu.
Tell him you need "carnada para pescar" (car'nadda 'pahra pes'cah). He will also point you to the right place to go. Cubans are very passionate about their fishing and love to share with tourists, especially knowing that being friendly will probably put some extra cash and goodies in their pockets. Just be discreet, as their Tourist Police is always watching them - they are not supposed to pester tourists for money or gifts. The visitors have to take a discreet initiative.
Before you leave, I suggest you go to A&N or WallyMart and get a couple of cheap spooled spinning reels and/or telescopic rods to take with you to use and leave as gifts. If not, just buy a few 300-1000 yard spools of monofilament. Nothing fancy - the cheap Korean or Chinese stuff is just fine, but go big - 10 or 12 pound test - even 15 is the ticket. Cubans rarely fish for fun - if they catch a fish, they want to yank it out of the water, kill it and eat it. If there are locals fishing with you, offer them the fish you catch - don't catch and release unless they don't want the fish or is an inedible species. (basic rule: the more colourful the fish - the less edible it is). You would also do them a great favour if you take a few bags of decent fishing hooks. I suggest you get the regular barbed chrome colour ones - size 1 and 2 are good. (Eagle Claw or DNE are just fine). Forget rapalas and other trolling tackle - Cubans are still not allowed to own private pleasure crafts. Only licensed fishermen can own boats under strict control and they already have their own gear.
Fish from shore where the locals do - you don't need to fish from the resort beach. There are plenty of better places, such as rocky points separating sandy beaches. Explore.
All that said, Santa Maria is heaven for tarpon (locally known as sabalo) and permit and a bunch of other species. Although December is not the best time of the year to fish for tarpon and permit, it can still be quite good if the weather cooperates. My advice to you is to take a couple hundred extra dollars with you and book yourself a guided tarpon and permit fishing tour. If you need a name or two of local guides who will certainly get you into some quality fishing, let me know.
A bonus not-fishing related tip: DO NOT buy cigars from your friendly local resort staff whose "cousin works in the factory". It is a LIE. The security in cigar factories is tantamount to that in the diamond mines in South Africa. They CANNOT take export grade hand rolled cigars home. All they can get is discarded seconds or even worse, leftover tobacco swept from the floor. It is crap - you are better off buying Swisher sweets than black market Cuban cigars. Buy only in the government store: Casa del Habano. Everything else is junk, garbage, waste of money.
I always laugh when people tell me about the super hot deal they got on their Montecristos or Cohibas. There is no such thing as a good deal. A 25-box of Montecristo n.2 will cost you at least 200 dollars in Cuba, which is still a bargain compared to 550-600$ - the price the real thing commands in Canada.
Oh, one more thing: if you happen to have an old cellphone or two sitting in your drawers that still have some decent life left in them - take them with you and give them to some person who has provided you with outstanding service (or whom you simply liked a lot). Cubans crave that stuff (it is too pricey for most of them to afford it). They can unlock any cell you give them.
Oh, I envy you so much. I wish I could take a Cuban vacation again this year, but it won't happen for some time. I was last there in March and had a blast driving around the island with my wife. Cuba - best place on earth for fishing, music and cigars! BON VOYAGE.