I have two sinks, of equal size, and both are deep - an important feature as you note. But we never use one of them, other than as a place to hold the dishes that are drying. For that purpose it is pretty convenient, because the dishes aren't going to be tossed about if you're underway. I suppose we could do without that 2nd sink, but then we'd still need counter-top space to place drying dishes, and they'd be more susceptible to being tossed around on the counter top. So I guess I'm happy having two, but it will be interesting to hear what others say.

btw we have a salt water pump at the galley for washing dishes, and use a spray bottle to rinse them, to conserve fresh water.

(edit) keep in mind the bigger (ie wider) the sink, the more water it takes to reach a usable depth for washing, possibly leading to more wasted water. I normally find a cooking pot and use that as the 'sink within a sink' but a lot of people will just fill up the sink to a certain depth. Narrow might be better, provided you can at least get your biggest pot in there. A high tap is also handy for filling buckets and tall pots.