Check your user manual before using those! The manual of the Garmin 60cx specifies never to use them. I believe that's because they give a bit more than 1.5V when new, which is bad for electronics not designed to take higher voltages.
Luckily, the 60cx has a good enough antenna to not lose satellite signal under my winter coat, so its easy to keep it warm, sliding it out to check position every once in a while...
Added : Otterbox and Pelican cases are rigid waterproof cases that come in various sizes, often used to protect electronic devices
Hmmm...can't honestly say that I have checked the manual for my 60cx regarding the use of lithium batteries, but we have been using them for over a year now without any problems. We were constantly changing batteries with alkaline type batteries. One problem with lithium batteries though is that when they go dead they don't mess around. Better have an extra set standing by (which is a good practice no matter what type of battery) because once those lithium batteries start to go, they crash fast.