"classes" of cars exist for the same reason "classes" of computing exists, e.g. servers, desktops, laptops, tablets, cell phones, etc. You can produce a more compelling product if you pick a very clear target, and you cover more of the market if you pick a variety of very different targets. This is super important for car companies, who can only afford to develop a few cars at a time.

As for boat-cars, I'm sure there's a few folks out there who would love one, but Tesla is seriously underwater. They need to penetrate the largest/most profitable markets and capitalize. Then they can worry about tiny niche offerings.