1 Isaac Newton: Granted my superior insights into the nature of the cosmos, I have constructed an apparatus capable of conveying us through time.
2 Edmond Halley: Mr Newton, yours is indeed and beyond a shadow of a doubt an intellect of the first order.
3 Isaac Newton: Here it is, Mr Halley. {it's a blue Police box}
4 Edmond Halley: Mr Newton, I believe this provides evidence for the aphorism that greatness is often linked with insanity.

Time machines have come in an amazing variety of shapes over the years*. Unlike, say, a boat, which has very definite real life constraints on how it should be built in order for it to achieve its primary purposes of (a) not sinking and (b) moving through the water, a hypothetical time machine has no such constraints, or at least not any that we really know of. Sure, it needs to move through time, but nobody really has any idea what sort of protrusions or flanges or other physical features a machine would need to enable it to do so efficiently, or without falling through an inadvertent time vortex or whatever.

The result of this is that boats always look pretty much like a boat, whereas time machines always look like... well, that's the point. They don't always look like anything. They can look like whatever the designer or artist wants them to look like.

* No pun intended.