Every other boat will be able to accomodate ECV's. A boat leaves for MK approximatly every 20min, so if you just miss the cruiser you could wait ablout 40 min for it to return.
The big boats that load on the left side of the dock take ecvs (be careful loading and unloading, they don't use a ramp and sometimes i've gotten a bit stuck with the gap of the boat to the dock).
The small wooden seat boats will not take ecvs.
I have stayed at WL using an ECV many times and never had a problem with the boats. I would say that at the WL dock the wait is usually well under 10 minutes. Going back from MK, it is a bit longer, but it really is a minimal wait. I think you will be pleasantly surprised.