Unfortunately, like many vehicles electric boats are limited to new... no one offers retrofit parts, and it will take decades before enough of these trickle down to new owners for them to take the place of existing boats, even assuming the drives last that long.

I'd convert my boat to electric in a heartbeat if I could retrofit the drive for a similar cost to a replacement engine/transmission... but that's still impossible, and probably will be for longer than I'll own it.

Most of the people on the road or on the water don't buy new for very good reasons. If we're going to limit climate change, there has to be more done for all those people than just telling them "buy new or go without". An effort needs to be made by the governments of the world to provide affordable conversion parts or replacement vehicles for those who will never buy new.