Nicaragua isn't the only place in the world with freshwater sharks, although I don't know of any other place that has landlocked freshwater sharks (as opposed to those who have ready access to the sea, like those in the Zambezi River and - I think - in Australia). But when I was living in Central America, the Lake Nicaragua sharks had a terrible reputation for being very aggressive. I don't know how much of that was the typical hype surrounding all kinds of sharks and how much was real, but they are a variety of bull shark. I have encountered small bull sharks in fresh / brackish water in a lagoon before, and they scared me because the visibility was terrible. They didn't keep me out of the water, but it was unnerving. And I would also expect the visibility in Lake Nicaragua to be lousy most of the time.
I assume you're talking about the Caribbean side of Nicaragua and Costa Rica; I was in Honduras for a while (a little more than a year), and know something about the southernmost part of Honduras where it borders on Nicaragua and points north (on the Pacific coast). I wouldn't look for much exciting diving on the Pacific coast.
*Every* Central American country has some beautiful places on the Caribbean side. Still, I would bet - and it's just that, a bet - that you will find better diving around Costa Rica (and more and better operators) than you will find in Nicaragua. Nicaragua's economy was devastated and tourism interrupted by the "recent unpleasantness," while Costa Rica has been a very attractive tourist destination for a very long time. I don't think you'll have a lot of choices in Nicaragua, and I would be very surprised if they compared favorably to your choices in Costa Rica. The one dive site I heard about in Nicaragua when it opened up again was described as very unspectacular (sandy bottoms, etc.). But that's second hand.
