St. John's River is unique among rivers, and not just Florida rivers. It's one of the very few major rivers to flow north in the world. And at three hundred miles long and often two miles across, St. John's River is certainly a major river.
It's also a very houseboat-friendly river. Since the elevation change is negligible (there's only a 30-foot difference between the river's highest and lowest point), the St. John's River flows very, very slow, and very, very wide. You'll have plenty to see while cruising at these extremely leisurely paces. The river winds through rich green basins, partly preserved through the state parks system, that birds flock to by the hundreds and thousands.
There's plenty to see in the water below as you float down these slow-moving and in various locations, incredibly clear waters. Manatees can be found near the Holly Bluff Marina at times, however, your best chance to see these gentle creatures exist within the warm spring waters where they spend their winters. If you're in a fishing mood, casting a line might bring you some excellent Largemouth Bass, which the river is known for. It might also bring you saltwater fish like Flounder and Sea Trout - or fish that thrive in both waters, like the American Shad.