Venetian Waterway Trail in Venice, Florida, runs along both sides of the Intracoastal Waterway. If you ride both sides of the waterway, they advertise it as a 10 mile ride, but it might be easier to bike part of the waterway and then use the new bridge over US 41 and continue your ride on the Legacy Trail (see below). Here's a map they have posted with info on the Venetian Waterway Trail (click to enlarge)...
The trail is paved and in pretty good condition everywhere. If you bike the western side, you'll leave the Intracoastal for a loop around a housing area and the high school campus, and then return to the water.
You can cross the Intracoastal Waterway to bike the other side by using any of the three bridges along the route. The so-called Circus Bridge, which is US 41 (the Tamiami Trail) near the southern end, and the bridge at the far northern end (which is also Tamiami Trail) both have wide walkways protected from traffic. The middle bridge, Venice Avenue, has a narrow raised walkway and has signs telling you to walk your bike. Here's a view from one of the bridges.
This is your view as you near the downtown (northern) end of the trail. The bridge is Venice Avenue which I had parked beneath in a small lot. I then biked the western side to the Circus Bridge at Tamiami Trail and crossed to the east side which goes a bit farther than the west side, and you can even gain a bit of extra mileage if you bike the perimeter of Shamrock Park which is at the terminus of the trail.
Other parking areas are at the old train station on the east side of the Intracoastal at Venice Avenue (south of the road) and another at the Circus Bridge at Tamiami Trail and Center, also on the south side of the bridge (there is a traffic light at the intersection.)
The trail now connects at the northern end with the Legacy Trail, thanks to 2 bridges over waterways and a monstrous overpass to get you across US Route 41 safely. This provides another 12 miles or so of biking.
My Legacy Trail post