I was at CES last week and saw the new Mercury motors in person (all 3 models) and I have to say, they're really slick.
The 7.5e is available now, with a 20e and 35e model both out by the end of this month. Two more are apparently coming for the end of this year, but Mercury wouldn't discuss further details.
The battery, at least on the 7.5e model, loads into the top of the motor and is removable, so you can swap it out for a second battery if you run low.
This is a sensible arrangement and given its size, there should be plenty of capacity. I was skeptical of my electric lawn mower when I got it, but it goes a long, long time with a battery not much bigger than what I have on my drill.
These are first steps, but it's pretty clear this is the future. I can tell you one thing - when you hit the throttle with an electric outboard it doesn't take a second or two to rev up to speed like a gas engine does. You get full throttle to the prop instantly, so the hole shots are just unbelievable. The motors are also silent, no fumes, no messing around with fuel, loads of advantages. I'm pretty encouraged by what I see.