Insert for the sink?
I haven't seen mention of an insert to cover the sink. It would increase your counter space, but then you have to have stowage for it when using the sink. If you go with the idea, you probably should caulk the joint between the sink and the counter, as the sink would now sit under the counter instead of on top of it. The sink mounting might be a little tricky, but I'm sure I've seen this on other boats, so it is possible.
Love the renderings.