You list this as a 25 "E" model which makes it electric start (at least it was when it started it's life).
"Older" and "Mariner" could mean alot of different things. Mariners are either Yamaha's with Mariner decals or Merc's with Mariner decals (the 25 was built in both flavours).
If it was the "remote/electric" Merc version the kill switch would have been either dash mounted (if used with a 2000 series controller for example) or on the shifter (3000 series etc). So, if this was/is a remote electric you need to add a stop switch - it never had one on the engine itself.
If this was/is an electric start "tiller" model the kill switch would be an on/off type located just to the left of the choke knob (a small sqare hole would indicate the missing part). Additionally, if it's a tiller model there is a stop switch (not marked and can't be seen just by looking) in the very end of the tiller handle itself hidden under the rubber (when you are holding the tiller you can use your thumb to "depress" the end - the handle is hollow and the stop switch is (inside) the tube.
The Yami/Mariners had a small (push to kill, rubber covered) switch that looks like the primer bubble on a lawnmower etc, located on the front of the lower cowl. I have never seen an electric start 25 Yami version so can't state if the kill switch was different, moved etc and my manual doesn't give that detail.
Hope this helps a bit - choking the motor to kill it really isn't a "good thing"..
