Perhaps a three-hour bus trip through the Yucatan jungle wasn’t the best idea to try and rescue their marriage. A bad American movie dubbed into Spanish with the volume cranked hid the silence between them. Gareth turned to look at his wife; she held the flat of her hand against her belly as though she still carried their child. For some reason it annoyed him. He looked back to the window, “You want to go on to Panquen after Chichen Itza?”

He saw her reflection in the window shrug silently.

Their holiday in Cancun had been a time of accusatory silences on the beach followed by bland meals under the glare of neon lights in their faceless hotel.

I like it. Nothing clearly speculative yet, and although it isn't inherently super interesting to me theres the "tension" of the relationship issues. And the more interesting suggestion that something happened with their child.

I'd take out the first line. Especially as a first sentence, it's kind of strong with the 'telling' aspect. And the rest of your prose does an admirable job of describing the tension between them. And you establish the setting in the dialogue; any further details about the length of the bus ride could come later.

Does Gareth's wife have a name?

Mexican meals bland? That goes against the stereotype strongly enough to reflect their emotional issues. I like it.