Agree with SBG about carrying a backup GPS. Beware of differences between your primary and backup unit. For example, if you use routes, be advised that the older/different units often don't support as many route waypoints (legs), so if you have long routes and copy them to your backup, some of the points may be truncated without warning. A few tribers have learned this the hard way... If you only have a handful of waypoints it doesn't matter.
I use a 78sc as my primary and a plain 78 as my backup. Each is in their own Aquapak (with 2 spare batteries, a piece of cloth for dealing with moisture when changing batteries, and a desiccant packet). That way, when one GPS fails, or the batteries die, I just turn on and switch-over to the other GPS unit. I prefer this to changing batteries on the water.
If I had this to do over again, I would get two 78sc units for the primary and backup. The more advanced units (78sc and 78) let you share data/waypoints wirelessly which might be a godsend when you discover that you are missing something on one GPS and need to transfer it from the other, while on the water.
I'm a fan of bagging my GPS. FWIW, I have never had a failure, over many years, with any GPS stored in an Aquapac with a desiccant packet. This includes a 76 unit that got me around Icleand and Newfoundland by kayak. I bag my radio too. A bag can make it tough to read the GPS at times, but it beats a dead GPS. Of course electronics can die at any time, so you still need to have the tools and know how to navigate without the luxury of a GPS.