Congratulations on your boat acquisition. From your post I am guessing this is an older boat. As you are aware, having 40 years of sailing experience, the builder could have used just about any make and model of micro wave that will fit in the space and was available during the build.
You have the same freedom. Measure the space (height, width, & depth) that you want to use to install the appliance then find a model that will fit and is at a price you want to pay.
I know of no one making battery powered microwave's. You’ll need to find one that works on AC, 110V, 15amps or less. You’re not limited to it being a microwave. It could be an air fryer, toaster oven or coffee maker if the space is a match. You will want an appliance that has adequate space so as not to over heat the surrounding surface.
Good luck with your project.