There is a post here (from 2013 with recent update) about an electric car ferry in Norway: https://endless-sphere.com/forums/viewto ... 39&t=47249
The designers stayed within the displacement rule by making it a long catamaran and not very fast. With a 500 foot waterline (262 x 2) speeds of 15kts are reasonable, at least in favorable conditions. I don't remember what the article said about electrical use, but at least conceptually, it's a good example of how to economically move cargo (cars are not heavy compared to the space they occupy) through water.
The working boats around here are often at a near-standstill during the fastest tidal flow (6kts) but one can pick one's tides, or wait for slack water, etc. To see how much power your boat would need in your waters, I'd look at commercial fishermen, who tend to be frugal. Some will need a fast boat to get out and back, 15-20kts, others will use a slow boat 5-10kts. Your tugboat friend probably knows this well: slow and steady wins the race.
I don't know where electric marine motors are sourced, but I expect you could find a wide assortment, new and used. Diesel electric power is a common thing on the water at least for auxiliary work. More important (IMHO), is the hull design and what you want to do with it. If the boat originally had two motors in it then it is likely to be a go-fast boat, and might be an awkward conversion to a single screw, if that's what you envision. But I suspect the motor would not be the expensive part, nor even two of them. Ah, I just remembered! There are charter companies who use twin electric motor catamarans for their charters. They're popular, I think... Here's one link I found: http://oceanvolt.com/sig45-electric-catamaran/
That's a 90 foot waterline, more or less, so likely less than 10kts cruising speed, but for a sailboat that's quite fast. Catamarans also have less parasitic drag from water (and less carrying capacity as a result), so difficult to transform that data into useful information for a 30 foot motorboat hull.
If I were in your shoes, I'd try to find the absolute minimum power my dreamboat would need in local waters (which in my case was a canoe and a 35 foot-pound 12volt motor) and then do a careful conversion to electric power (and stretch the boat longer if possible). One thing in your favor is the weight of batteries. Within the displacement rule, more weight means more waterline (as the boat is pushed down) and faster speeds. Outside the displacement rule, the bow needs to rise and at least attempt to climb the bow wave, so all weight needs to go aft (or overboard).
Good project, I think! There are electric outboards on the market, but generally most of the recreational marine industry is tied to twelve volt lead-acid batteries and either gasoline or diesel propulsion. You knew that, I'm sure, so am only mentioning it to point out that it's a fairly clear horizon for any entrepreneur or wiseguy inventor Brainiac-type.