[CR Yaker]

You'll love it. It's rainy season now. In fact, here in Jaco it just rained tonight after a two week drought, so it's back on. Should start to see more big thunderstorms. But that's always later afternoon to early evening. Plenty of sunshine the first half of the day. Probably even more so in the Guanacaste area. Hopefully your flying into Liberia because of the active Volcano effecting the San Jose airport at times.
I think Samara is your better beach area for access to fishing. There is a kayak outfit there, look online.
I wouldn't bring too much equipment. A two piece rod and reel is fine. Maybe a few lures like a surface plug casting in the rocks, see if a Rooster will attack it.
The best shore fishing is live bait for Corvina or Snook. Not sure what the river situation is there. You could also try bottom fishing off the surf with some shrimp for some African Pompano.
A sabiki set up might do well for catching bait during the lower surf. Fish mid high to high for best results.
You might ask one of the locals, they usually net at night, to take you out if your Spanish is good. Of course they are going to give you a Gringo price, but if your lucky you could get out half a day inshore fishing for $100.
Most import just relax and enjoy.
Pura Vida!!