In the town of Merida on the Island of Ometepe you'll find Caballito del Mar. Caballito is a restaurant, a hostel, and the launching spot for kayaking tours of Rio Istiam.
This tour was amazing! I had the opportunity to see caiman, members of the crocodile family, in their habitat, which was a dream. Don't worry they aren't going to get in your kayak.
The easiest way to get to Merida is to take a bus to Santa Cruz. You can either walk the 5km to Caballito or wait for a bus which might not come. That or you can pay a lot and rent a private taxi or van.
A three hour guided tour will cost you $25 on your own or $20 if you're in a group. You'll come back from the tour hungry. A whole fish lunch complete with refresco will set you back $6.