I grew up as one of the first in the generation to have children's TV and fondly remember being a member of the peanut gallery, and remember Chief Thundercloud and all the rest, however the association I recall best for the phrase 'cowabunga' is actually the Charles Schultz's Peanuts character Snoopy in his "Surfer Joe" persona where we'd see him, wearing his baggy baggy surf short, hanging ten from his surfboard while on the top of his doghouse as it transformed into an imaginary wave. The Ninja Turtles were relative newcomers and were no doubt paying homage to Surfer Joe Snoopy. Cheers.