I've ran a 1984 35hp Johnson since it was new on my 16ft boat, almost exclusively in salt and brackish water but I do flush the thing out after every trip. What it does see a lot of is sand and debris, but these motors are designed to deal with a good bit of that stuff without issues.
On mine, in a tub of water, I get water out of the tell tale almost instantly, long before the thermostat opens.
I change my thermostat everytime I do an impeller, about every two or three years.
Pulling the power head is a pain on any motor, and its worse on an older motor which will likely have some frozen hardware to deal with. One thing I did to mine when it was new is to pull out every accessible bolt and coat it with Never Seize to be sure I never have an issue.
I also keep a few spare motors around, grabbing them up anytime I find a deal just to have. I picked up a super clean '85 30hp Evinrude just last week off CL for next to nothing. It runs perfect and it came out of PA, a good distance from any saltwater. I'm in the process of going through it now to be sure its ready if the need arises.
As a note, none of the smaller motors work well on the ears like that, they're okay for some larger motors and stern drives but a bucket or barrel is the only right answer for a portable motor.
