Bluetooth and Wifi range onboard a boat
A couple of thoughts.

In a home scenario, the 2.4 GHz signal can reflect and propagate somewhat indirectly off flat surfaces.
There are also metal pieces which can sometimes act as passive antenna elements - wiring, pipes, construction studs and others.

On the boat, there are no flat surfaces, so fewer opportunities for reflection - any reflected waves will be scattered. There's also not as much metal on the boat to form antennas for re-radiation.

But I think the biggest difference is that the boat is in water, which absorbs the 2.4GHz signal very effectively rather than reflecting it. More about that here: https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water

So, it's the water, and a bit more.