add Hand Sink project

Are you planning on using raw water or fresh water (with a tank)?

Personally, I'd keep it out of the cabin - it's just easier, all around, to keep it in the cockpit. You could do it as simply as not having any drains and just let the water fall into the deck without even using an actual sink. If you think about it, what do you really need the sink for? All you need is a small faucet, tucked under the gunwale (there are some that swing out, too). Electric is nice as it frees up both hands. A fresh water sprayer could be another option. If you do want a sink (and an actual drain), I'd just install a new thru-hull as it would probably be simpler than getting at the fish box hoses.