The 60Csx has a barometric altimeter. There is a way to calibrate it, but I'm not familiar with how to do that. Check your manual.
Also, with the barometric altimeter turned off, altitude is the least precise measurement a GPS makes. It can be off by up to 100 feet either way. Usually it isn't, but you have to watch for it. I was just in Hawaii last week and with the ocean lapping at my feet, my Oregon said that I was at -52 feet (and +30 feet in roughly the same spot a day later).
I was also in hawaii where I first thought the elevation was off- maybe sea level is different in Hawaii due to the earth being eliptical?