Well, I figured out a few things:
The wiring harness on the port side of the ski actually goes into a heat sink, maybe a rectifier (don't have the manual) that is clipped into place. The lower clip has to move first, I was not able to do it by hand, but was able to push it back far enough with a screwdriver to pop the heat sink / rectifier loose with my hand. Installation is the opposite. You slide under the upper clip first and push it in to click it in place under the lower clip. The wires are all connected to the heat sink / rectifier and the entire assembly moves out of the way.
Fuse box. A whole 'nuther story. There are 4 clips on the fuse box. Two of them are in a bow/stern orientation, two in a port starboard orientation. Using the same screwdriver, I went under the back clip in the bow/stern orientation and proceeded to break it off. You cannot see the back clip behind the fuse box, maybe a mirror attached to the gas tank somehow will allow you to see the back clip; I just used "feel". Bad idea. It is now somewhere in the bilge. And, they are not the right clips anyway. They are for the fuse box cover, which now has to be replaced. The correct clips are the ones on the side with the port/starboard orientation. Pry them back - Just a little - and the fuse box pops off. The main wiring harness goes into the bottom of the fuse box. Now, the positive side battery cable goes to a stud that is buried under that wiring harness. It has another red cable attached. I can barely touch it with my hand, so, I suspect I will need an extension to get to it. If I go that route to remove the battery holder from the ski, that is.
So, I was able to move the back half of the battery holder into the bow compartment and place the battery into that half first. Then I put the front half over the battery and had just enough space and leverage to squeeze the two halves together. I also used a clamp. The kind with a squeeze handle and could squeeze it together. When I released the pressure, however, the halves separated by maybe half an inch.
Next, I will use a small C Clamp on both sides to squeeze it back into position and thread the hold-down bolt back through the holes and into the base. There are some holes that line up on the sides, I may put a nut-bolt through those instead. There were none there.
I'm sorry I have no pics, I was by myself and needed some extra hands and arms to pull that off.
So, I will order a new fuse box cover and decal, get some stainless nuts, washers and bolts and go at it again.
Think I will sell the ski before I do this again!