So you're a videogame now?

Yes

Vroom Vroom (not even sure what kind of noise a nuclear powered car might make)

These tricks are still limited. Transform the PC into a boat and you'll lack the ability to have that PC move around on the boat while it's moving. That might not be needed, but the hat trick and things like it aren't going to work well in every situation. In Fallout 3 the trains needed to remain fully grounded, but I'd think a boat ride would be a choppier experience. Unless the sea is incredibly calm, that boat's going to be rocking around - and the less than perfect cart physics could actually help rather than hinder the realism as long as the boat remained at least the right side up.

Yeah but it's not necessary to have the player moving on the vehicle. You normally don't want a vehicle for moving inside of it, you want it to move outside. So you can easily discard any inner visibility and simply transform the player into a box that moves around, changing the controls a little bit. If it's a boat I have no clue how they can do it, perhaps following the same basic concepts of waterwalking.

In that one story quest Dogmeat was not forced on you. He was a mandatory part of the quest, sure, but he didn't take up your "companion" slot unless you decided to team up with him. If you didn't, you and your actual companion just treated him as a mobile quest marker. It's usually me and Nick following Dogmeat - so if Dogmeat's given a similar role I can't see why Codsworth couldn't come along.