Sharksuckers Echeneis naucrates are a member of the Remora family of fishes. Unlike the species known as a Remora, Remora remora which has no distinguishing markings except for some speckling, the Sharksucker has a much more elongated body, and juveniles have a black mid-body stripe bordered by white that diminishes to just the head area as the fish matures. Remoras tend to prefer manta Rays and Sharks whereas the Sharksucker will also attach to large fish and sea turtles. They do not just attach to the host in a permanent place, they scoot around the host's body quite easily. The suction disc on their head is an extremely modified fore-dorsal fin. So, yes, in the pictures with the Sharksuckers on top of the turtle's shell you are looking at the Sharksucker's belly.
The common remora Remora remora is a pelagic marine fish  belonging to family Echeneidae. The dorsal fin, which has 22 to 26 soft rays, acts as a suction cup , creating a vacuum  to allow it to attach to larger marine animals, such as whales, dolphins, sharks, and sea turtles. The common remora has a suckerlike dorsal fin and an anal fin. Its body can be brown, black or grey in color.
The white sucker Catostomus commersonii    is a freshwater cypriniform fish inhabiting the upper Midwest and Northeast in North America , but is also found as far south as Georgia and New Mexico in the south and west. The fish is commonly known as a "sucker" due to its fleshy, papillose lips that suck up organic matter and aufwuchs from the bottom of rivers and streams. Other common names for the white sucker include bay fish , brook sucker , common sucker , and mullet. The white sucker is often confused with the longnose sucker C.