I think everyone wants you to get a caiman lizard. Really, next to the marine iguanas of the galapagos, they're the only lizards (that I can think of) that will dive deep down into the water. The Tribolonotus I think someone mentioned are really not that aquatic, more terrestrial really.
I think your best bet is the Shinisaurus. They're about 12 to 14 inches as adults, though I'm not entirely sure they'll dive deep into the water. I even had asked this very question to one of leading Shinisaurus experts in the US, and he wasn't even sure. They're often just kept in slightly swallow, stream type setups. The few I worked with also were pretty skittish and would bite if mishandled. They have a nasty bite too!
I think your best bet is the Shinisaurus. They're about 12 to 14 inches as adults, though I'm not entirely sure they'll dive deep into the water. I even had asked this very question to one of leading Shinisaurus experts in the US, and he wasn't even sure. They're often just kept in slightly swallow, stream type setups. The few I worked with also were pretty skittish and would bite if mishandled. They have a nasty bite too!