If your Honda 8 hp is manual start it will put out only 6 amps from the charging circuit. If you have the electric start option iit is 12 amps output. If your group 24 engine start battery was discharged to. a point to where you could don't start it, with the manual, it would take at least 6 hours, and with the electric start about 4 hours.
A li ion battery jump pack is a very good investment, and will start an outboard. Mine will start an 8.3 L diesel...
But you are really best advised to have a second 12 battery. The group 24 will weight 45 to 50 lbs. If you run anything other than basic LED lights (and you may run your anchor light at night, if anchored--actually a good idea to use even with bow to beach, since you should have both a bow and stern anchors out...
I would not consider going on a trip on Lake Powell without a second battery. The reason is that if your single battery goes bad--and even new ones can, you are really out of luck You have to have a battery to run your engine, and electronics.
Yes, jumper cables will probably start the engine. why not wire that second battery in and have the switch and a voltage sensitive relay? Then no worry.