Natural gas is great because it can be ramped up and down quickly to respond to need, whereas nuclear and coal are more fixed. Hydropower can also be ramped up and down quickly.
However solar and wind aren't as intermittent as you might think, because the electrical grid can smooth out shortages in one area by bringing in electricity from another. Advances in grid technology will open up other options for regulating power, like demand pricing.
I can imagine systems where you'd plug your car or boat in, but set it so that it won't start charging until the price of electricity drops below a certain threshold (which would probably be sometime in the middle of the night). High electric consumers like aluminum manufacturing would vary production based on price.
Exactly. Not only does production tend to balance out over larger regions but demand does too (i.e., California uses a ton of power for AC in summer, when power demands are relatively low in the PNW - summer is when we generate a lot of solar in the NW and don't need all of it. similarly CA is still producing solar power in winter when we need it for heating and they don't so much). We just need a good grid to move it round.
Also, charging your car at low-demand times is doable (and done) now. That makes a ton of sense where the base power production is on all the time but not heavily used at night.
As others have noted, the range issue has largely been solved for EVs. A 300 mile range is more than enough for almost anyone so long as rapid charge stations are available along highway for long road trips, and they are being installed fast (especially if you own a vehicle made by the "half-baked startup" that someone mentioned, which is building its own network). We have a Leaf with a 107 mile range and even that is enough for almost everything we do in the urban area.
But the difference between cars and boats, and the real problem for electric boats, is that by the nature of a car you are almost always going to be somewhere you can recharge on a daily basis. Not so much the sailboat. It's hard to imagine using one for anything but short trips without some way to recharge, like a genset. That in itself is not horrible - you would in effect be operating like a plug-in hybrid car, which saves a ton of fuel. I know someone with a plug-in Prius who gets gas every 2-3 months.