A healthy tortoise is very active. If he isn't marching around a lot...he probably doesn't feel well.
Take him to the vet if he has the following symptoms: "If the animal is blowing bubbles out of its nose or mouth. If the animal is wheezing. If its eyes are swollen shut or will not open. If an aquatic turtle is swimming in a lopsided manner. If there is blood in the stool or urine. If there is an unusual swelling anywhere on the animals body. If the shell has white areas or weeping areas on it or appears to be eroding on the edges. If its droppings are or a constant liquid nature rather than firm. If the animal appears lame when walking. If the animal will not eat. If the animal's shell has been compromised by accident or animal attack" http://www.tortoisetrust.org/care/faq.html#water