Can a DSLR or mirrorless camera body be upgraded with a higher-resolution sensor?

In almost all interchangeable-lens cameras, no: the sensor is not a practical upgrade part. The camera is designed around that specific sensor, including firmware, image processor, autofocus, metering, and sensor positioning. Even if a different sensor could physically fit, the camera usually would not recognize or operate it correctly.

The main exception is some medium-format systems that use replaceable digital backs.

For typical DSLR or mirrorless bodies, getting more resolution generally means buying a different camera body. If your concern is an expensive underwater housing, check whether a higher-resolution body with very similar dimensions and controls may still fit the same housing or have a compatible housing option.

If changing bodies is not possible, one workaround for static subjects is shooting multiple overlapping frames and stitching them into a higher-resolution panorama. Using a longer lens can also help if your real need is more subject detail rather than more native sensor pixels.