re: Has anyone been fishing in Nicaragua???

bluemoons has some good advice in their reply!
Nicaragua is an amazing destination for outdoor adventure and as bluemoons mentioned, there are many different habitats to fish. You should easily be able to find an exciting way to wet a hook no matter what your skill level.

Lots of people unfamiliar with the situation in Nicaragua (like KemoSabe) have painted a picture of doom and gloom that has kept would-be-travelers away for almost 2 years and destroyed the tourism economy. As a result, your dollars will have more buying power! Lodging will be readily available and you won't have to worry about crowds at bars, restaurants, or surf breaks.
The result of far fewer tourists, and greater competition for their business has, no doubt, created some tense situations. Crimes like vehicle burglaries and petty theft may have increased (per capita) but violent crimes against tourists is still rather uncommon.
The political unrest has forced the cancellation of the Flor de Cana tournament for the past two years, but the fish don't seem to know it.
I have had a few great reports from the north (Corinto & Aposentillo) but I don't know anyone in the southern half of the country who fishes regularly.
Just about any village on the coast has someone who will take you out & put you on fish, but without a few more details on what you're looking for, it's rather difficult to recommend something.
It really depends on your budget, your skill level, your time, and how well you speak Spanish.