I have a 19 month old and he is the same way. He says mama, dada, cheese, water, ball, food, and car. Most the time though I am the only one who knows what he is saying :)
Talk to your pediatrician and see what s/he thinks. They might suggest an evaluation. Is there more than one language spoken in your home? It can take a little bit longer for children to develop vocabulary, but they learn to speak both.
I wouldn't worry about it. My little girl was the same way up until about a month ago. She's 23months now, well almost! :) Then it was like overnight and she was stringing 3 and 4 words together! The books I've read said that early talkers use one word at a time and their vocabulary is HUGE! Late talkers usually skip the one-word business and go straight to sentences---like mine did! One day I turned around on my heel because my daughter had repeated what I said: "Tha Naw Nie!" (That's not nice!) Forgive the spelling as I'm unfamiliar with Baby language...hehee
Talk to your Pediatrician for additional info...
Good luck!