Re: '71 merc 135hp won't shut off!!??
Here's some examples of marine-grade, sealed electrical switches that would be appropriate for your application:
http://www.wiringproducts.com/index1.html
First you'd need to locate a place on the motor to mount the switch. Choose a location somewhere on the front part of the motor so you'll be able to reach it from inside the boat. Then drill away (cringe!) and mount the switch.
Once you have the switch mounted, disconnect the +12V white wire at the side of the CDI box and connect it to the switch. If it's not gonna reach, splice in a length of wire. Be sure to insulate any splices you make.
Then run another wire from the switch to the same terminal on the CDI box where you just disconnected the white wire.
Note that normal convention for the switch would be "open/off" is switch lever down, and "shut/on" is switch lever up.
That's about all there is to it! You'll need a set of wire strippers/crimpers and appropriate crimp-style wire connectors to match the switch's connections. And some shrink sleeving over any exposed splices or connectors helps to keep things tidy. This electrical stuff is commonly available at your local Rad Shack or most auto parts stores.
HTH, now you can Git 'er Dun!!!!!!..............ed