Hi Sean,
I have the ETrex Lengend Cx (color screen) also. In retrospect, I wish I had bought the 205 or 305 for the compact size and HR feature. The Legend is a bit bulky, especially on a ski. I never use the map features or download capability, as I'm Mac based and everything out there is PC compatible. This is perhaps the greatest disappointment, as essentially what I have is the glorified equivalent of a bicycle computer. If you go to the option for displaying four readouts, the display is huge.
As for mounting, I purchased the Garmin marine mount and use the 3M 'reclosable marine mounting system' (like plastic nubbed Velcro), which provides a positive 'locked in' feeling of mounting plate to boat. Since my S1-R has separate footwells, it sits on the center divider about mid calf; easy to reach the buttons, etc., and loop the cord around a footstrap in case it pops off. I can also switch it from ski to EFT with the 3M attached. Only knocked it off once in big conditions, but I do rub against it from time to time.
As noted, seal the battery compartment with plumbers grease or Vaseline. I also used electrical tape around the opening to seal it completely from leakage.
"Nice? It's the only thing," said the Water Rat, solemnly, as he leaned forward for his stroke. "Believe me, my young friend, there is nothing -- absolutely nothing -- half so much worth doing as simply messing about in boats."
'The Wind in the Willows'~Kenneth Grahame