It depends on the liveaboard. On the high end boats (Aggressor, Peter Hughes, Mike Ball, Nekton, Bilikiki...) this is not the case. Let me give you an idea of Aggressor (I have been on Fiji, Palau, and Belize since November 2003).
The boats are small so you are sharing quarters with other divers on bunk beds.
Most of the cabins are double occupancy w/bunk beds, with a double bed on the bottom bunk, and a single bunk above. There is sometimes one quad cabin on a few boats, but this is the exception rather than the norm. They ARE small, but have a private bath including shower, A/C, and in room video. Linens are changed every 2-3 days, towels every day. Beds are made up every day during the first dive.
After a few days of being cramped for space people start undressing in front of everone else.
Never seen it (darn)! Heard about a group from Russia that were a bunch of drunks and they behaved this way in Fiji, but I have never seen it or heard about it otherwise on the high end boats.
Some people get irratated with others, etc.
Maybe I have been lucky, but this has not been a problem. Sometimes there is a "boat character" but it was never real bad (unless you are a particularly intolerant person.) Quite the contrary, I have met a lot of people that I still correspond with on these trips.
Plus, I snore really bad which would probably bother anyone in close vicinity of me so I wouldn't want to inconvience them either.
This would only be an issue with your roommate (who is your girlfriend who, I presume, is used to it.)
What is the week like? After a day or so, your fellow divers seem like long lost friends. you wake up, have breakfast, dive, have a snack, read or nap on the sundeck, dive, have lunch, read or nap, dive, have another snack, read or nap, dive, have dinner, watch a slide show or video, night dive, have hot chocolate, watch a DVD or read or whatever, go to sleep. Repeat times 5.
Common areas on the boats include inside lounge (w/TV, video, DVD, stereo), sundeck with shaded portions, and hot tub. Dining area also available to spread out during non meal times.
As you go down in price, maybe some of the issues you mention occur (I have heard some of the smaller Asian boats may be more cramped.) But again, if you make friends easily and can enjoy a week of eat, sleep, dive, and relex, then the top end liveaboards will be as described.
Go to the Aggressor website (or Peter Hughes) and you can see photos of the boats and diagrams of the boat layouts.
