I use a 55 pound thrust (12-volt) on my 14 ft tinny and it will push it at 1.5mph for about 6 or 8 hrs if things are calm. Fighting a wind or current, that diminishes. On bigger boats like your 20, you present a lot more resistance to the wind, so you might consider stepping up to a 24-volt 80 thrust.
There are some that are really slick, in that they mount right on your main motor's cavitation plate so you can control them with your original boat steering (by the way, there's one on E-bay right now that's a real steal at $500). I can't use it because I don't have any way to recharge a battery at my marina or I'd buy it. They retail about 800 and this is a new one for 500.
Anyway, whatever you choose, you'll need to go more than a 12-volt to get a full day's fishing out of it. I really do love mine on the tinny, it's super quiet but it won't work on my 20-footer. The next best is a 9.9 gas kicker. That's what I use on my 20-footer.