I have had Garmin hand helds, I suspect since they first came out...Most recently the 76. But this 78SC is such a good deal, I'll get one on "black Friday"..
The 62 and 78 are basically the same spec wise--except the 78 floats as well as being "water proof" (I would not put it under water for any length of time!). The 62 does have the buttons arranged perhaps more conveniently for one hand use..
The 78SC has all of the US Coastal Maps, you can load lake maps, road maps, or topo maps from SD cards. The 78 can be operated with gloves, and also makes a great hiking instrument with its topo maps. (I also used like to take a topo map and compass--after all, that is all we had just a few years ago.) But in today's world, one can put all of the route in the GPS.
I like all of these because they use AA batteries--and all of the items in my ditch bag use AA batteries (flashlights, 2 meter radio, VHF radio, Strobe light, GPS) And I keep at least a dozen fresh AA batteries in a baggie in the ditch bag.