One reason for NOT doing that is because Kraken/Serpents don't get their water bonus when they are on a shipwreck just as they don't in a city/ruin/temple. So the image tells you whether or not they are getting their water bonus.
Hyhyhy You shall try it with your ranger, seeking for easy ruin defended only by a serpent.
Makmak, Vicotnic and me have actually checked that waropedia is correct here and water bonus does apply on shipwreck. http://www.warbarons.com/wiki/terrain