I know that you do not for the Illuminations cruise. You leave from the dock by the Yacht Club. And, I think that the other one leaves from one of the hotels (the Poly) that is on the Lagoon, but not absolutely positive on that one.
I don't think so. The boats are pontoons and IIRC, you needed to step across a bit of water into them from the dock. I didn't notice a ramp or any other way to enter. I also don't think there would be room for even 1 ECV on the boat. It is possible that there are boats/ways to accomodate ECVs, though, so it wouldn't hurt to call and ask. If ECV occupants could be assisted into the boat, perhaps the vehicles could be left on or near the dock. I'd call before the booking date just to see what might be possible.
You might also post this particular question on the DisAbilities Forum. Perhaps someone there has specific experience that may be helpful to you.
Here is a link to lots of general information about the cruises:
We did an Illuminations Cruise 5/30. We were picked up at the Boardwalk dock. Other boats leave from the YC/BC dock. We paid at the time of cruise ($120 plus tip). The pontoon boats will hold up to 10 adults & 2 kids. They have another boat (the Breathless??) that holds 7. It's a little more expensive.
DFIL was using an ECV for this trip. DH & the boat driver helped him board the boat. The EVC was left on the dock. There really isn't an space to put even one evc on the boat unless you had a very small party.