We have a 35' sailboat with a 11'6" beam. This means that the fattest part of our boat is 11'6" wide but most of it is narrower. As you can see from the aerial shot at the top of our blog, there is the pointy bit at the front and after her wide "hips" her butt is a little narrower.
As an aside, a sailing saying that tickles me is: sailing is simple, just keep the pointy end forward and the stick side up.
For us, the answer is no, it isn't cramped for living except when we are both trying to get ready to get out the door or when we are tearing the boat up for a boat project/renovation and we have things piled everywhere.
If you are over 6' tall you would have to hunch in our boat, but we aren't. We can see each other in almost any place on the boat from almost any place on the boat except the bathroom. Strangely enough, we like this.
This is a schematic of the interior of our boat. You can click on it to go to the larger size if it is easier to see.
We sleep in the pointy end which is 6' wide at the top and "feet are touching" at the bottom but also 6' deep. If you take a king sized bed and cut it into a V, you have our bed.
The middle section is a small bathroom (head) and a good sized (for a boat) sitting area. We can easily feed 6 for dinner and could squeeze 8 in if they were friendly.
Then we have the kitchen (galley) and the navigation table/desk.
The back of the boat is mostly outdoors storage and the interior stops except on the port side (the left side) there is a cave like guest cabin. Something like a three person tent that you have to crawl into. This is where our guests get to stay when they come visit. Let's just say that it is a good thing we spend most of our time outdoors for their sake.
The time that it does feel cramped is when we are trying to get something out of a storage area that is buried beneath something else. See "boat moments". Because we have to be so efficient about storage, things are stored in layers from the most often needed on top to the least often needed below or in a difficult to reach space. This is when we miss big closets and garages most.