If you follow the magenta line while running the ICW across the mouth of Tampa Bay, to Boca Ciega Bay, the indicated route would take you under the center section of the Sunshine Skyway bridge, and then back under the Misner Bridge, the northernmost part of the Skyway. Talk about going around your thumb to get to your little finger. I don’t know of anyone who actually does this.
Rather, the vast majority of cruisers use the “Skyway Channel,” which parallels the Skyway’s western face. The only trouble is that there is a bit of shoaling along the southermost part of this passage. The posting below is the latest on this page to indicate this skinny water.

Hi Claiborne,
I just got back from a week long trip from Clearwater Florida across the Lake to Stuart.
Items of note:
The Sunshine Channel just south of St. Petersburg and west of Sunshine Skyway Bridge had a low water depth of 6′ at southern marker “5”, otherwise plenty of water.
Mike Dickens