When you consider this, it's important to understand the difference in physics between moving a car and moving a boat, as it greatly impacts the power requirements and expected performance.
Moving a car takes a bunch of energy over a short time to bring it up to speed, very little energy to maintain speed, and an opportunity to recapture energy (regeneration) when slowing back down. There are a bunch of variable, but that's the basic cycle and principal.
A boat is very different. Moving a boat takes constant, continuous energy, and a lot of it. There is no "coasting" at cruise speed, and certainly no recapturing of energy. As a result, it takes much, much more stored energy to move a cruising boat in any sort of typical use vs running a car in typical use.
To put some rough numbers around it, it takes 15hp or so to maintain 60 mph in a car, yet your boat takes about 30hp to maintain 7 kts. So to travel 10nm it takes about 3hp-hrs of energy to move your car, and 42hp-hrs to move your boat the same distance. So over 10x the energy to travel the same distance, not considering the time to travel that distance.
You also need to consider recharge time in the context of commonly available shore power connections. A 3 hr cruise in your boat at 7 kts will consume about 100kWh of electric power. To replenish that from a 50A/240V shore cord will take about 10 hrs running at 90% load. A 30A/120V cord will take over 30 hrs. And that's just to recover from a 3 hrs cruise.
To deal with this, where electric propulsion provides acceptable results is for moving short distances, at slow speeds (e.g. sub 4-5 kts), hopping between docks with good power available. In another thread a Greenline owner described doing just this in the European canal system, and it sounds like a great fit. Commercial applications have been with short haul ferries, and have included huge terminal infrastructure for recharging in the available time between departures. Also puttering about the harbor in small day boats, including rentals.
I'm sure electric boat applications will continue to evolve and grow, but it's a very different problem to solve vs cars. For better or worse, a lot of boat and boat equipment manufacturers market to the perception/assumption that what's good for cars must also be good for boats.
Personally I think the real opportunity on boats is with a hybrid house power system. Big batteries, big inverters, big alternator, and a generator with auto-start works really well. We have house power with little to no load management 24x7. If we are anchored at night and underway every day, we never run a generator. And if we are anchored for multiple days as we are now, the generator kicks in once a day for 2-3 hrs and it's done. Add solar and you can further reduce or eliminate the generator run time.
