He already holds the world record for surfing the biggest wave on the planet at 90 feet (below) -- but pro surfer Garrett McNamara still thinks he can shatter it ... with his incredible customized oxygen-enhanced wetsuit.

Garrett tells TMZ, the suit was enhanced with a special oxygen pocket developed by Camelback -- the water canteen company -- designed to hold a small oxygen reserve in the suit's lining, allowing McNamara to breathe oxygen underwater through a straw.

According to Garrett, the suit would give him several extra minutes of survival in case a massive wave buries him below the surface -- and in those situations, minutes can save a life.

McNamara tells us, he's currently looking to break his 90-foot wave record with the new suit -- hunting down a 120-footer in Nazare, Portugal ... where the biggest waves in the world break.

McNamara says he uses military-grade wave-tracking technology to follow the swells -- the same tech used by the Navy.

He even has a private jet at his disposal -- to make the 5-hour flight from Miami to Nazare at a moment's notice.