What is the most practical way to categorise electric boat designs that can be amateur-built?

These craft fall naturally into 3 groups, which in turn makes it easier to sort out just which motors, battery voltages and controllers are suitable for each group

Small & Car-toppable

Boat dimensions	12 - 16 feet
Unladen weight	60 - 130 lbs
Load	250 - 600 lbs
Battery weight	60 - 120 lbs
Motor type:	Trolling motor - outboard/submersible
Motor horsepower	1/2 - 1 hp
Speed	1 - 3 mph
Range	3 - 4 hours, depending on speed and current/wind

Essentially this type of craft is the one- and two-man powered canoe. Small enough to be carried to the water's edge, but large and strong enough to support a trolling motor, one 12 volt battery and the picnic hamper.

Medium & Trailerable

Boat dimensions	16 - 25 feet
Unladen weight	400 - 1000 lbs
Load	700 - 1500 lbs
Battery weight	120 - 240 lbs
Motor type:	Permanent magnet - inboard
Motor horsepower	1 - 3 hp
Speed	up to 6 mph
Range	6 - 8 hours, depending on speed and current/wind

This is the two- to four-person launch. Heavy enough to need its own trailer (and a suitable launching site), but something in which you could comfortably take your family, or some friends, for a day's excursion.

Large & Moored

Boat dimensions	25 - 35 feet
Unladen weight	2000 - 3000 lbs
Load	1400 - 2400 lbs
Battery weight	240 - 480 lbs
Motor type:	Series wound - inboard
Motor horsepower	5 - 10 hp
Speed	up to 7 mph
Range	50 - 100 miles, depending on speed and current/wind

This size of boat is going to carry 6 to 12 passengers in some comfort. More importantly, the batteries can be used as the boat's ballast and there is less penalty in having a 48 volt motor and sufficient energy reserve to travel a good distance.

All these manufacturers have web sites where you can further investigate the design types:
Black River Boats
Glen-L Marine
Cobalt Marine
Duffy Electric Boat Co.
Beckmann Boatshop Ltd