electric GIS?

The best is a relative term. A GIS will move with the smallest of trolling motors, which typically are in the low to mid 20 pounds of thrust range, which about 1/4 to 1/3 a HP, depending on who's math you want to follow. This will push you to about 4 MPH, as long as you have battery life, which is the butt kicker with electric systems. A standard group 24 battery with 100 amp hours will suck down to needing a recharge in a couple of hours at WOT. If you reduce speed, you'll get more run time, but also will not go as far.

This said, you could opt for a big 12 VDC trolling motor (50 - 55 pound), which will push maybe to 5 MPH (it's a prop size thing), but you'll have a lot more power, if you encounter waves, wind and contrary currents. This is about 3/4's to a full HP and the GIS will putter along nicely, but it'll also drag down a group 24 pretty quickly, so bring two, one for the way out and when it slows dramatically, switch and return to the dock.

Of course, two group 24's and a 50 pounds of thrust motor, will weigh more then a small gas powered outboard (maybe 100 - 110 pounds) including a small fuel tank and worse yet, is you can operate the small gas engine all day long, without having to charge anything.

You can get performance props for some trolling motors, but these are limited. Some have had good success with making their own props, so they get the most from their limited power and battery life. Doing these things might get you to 8MPH and possibly add 50% to your range, but requires a fair bit of experimenting.