Wat Saam Prasob is the remnant of the old settlement flooded when Khao Laem Reservoir was created. In the dry season the brick shell rises clearly from the mud and shallow water; in wetter months you may see only the upper walls.
Boatmen in town can take you out when conditions are right. The visit works best as a lake trip rather than a stand-alone sight, especially if you go early or late in the day.