Is the boat you are using the one in your avatar? A 20 hp is probably more than you need--it is just a back up--a 9.9 would probably do the job, after all it sounds like a back up and it is just a back up that will get you in to somewhere safe.
If you do not want a back up/repatition on board your boat and you want to count on sea tow or whoever when you get in trouble--then that is up to you---but if she is going to break down--it will be at the most inopportune time--and waiting for a tow boat may not save you or your boat---but do as you may
