You don't end up spending much time in your cabin except to sleep, so lots of room isn't a priority. The central salon area and dining room is a comfy place to relax, read, visit and eat. Boats have books and movies available onboard along with the TV and VCR and a photo light table. Many boats motor at night, changing locations to set up for the next day's diving. Ask where your cabin is located in relation to the engine room.
Some cabins can be noisy and warm from that close proximity.
If you're prone to seasickness, be sure and take along medicine. The first night out can be queasy for those without sea legs yet. The larger vessels are pretty stable in the water, and even in a storm. I've been rolled back and forth in my bunk a few nights, the luck of the draw in hurricane season. Overall, you will find accommodations very comfortable, clean, and conducive to enjoying a relaxing week jumping in and out of the water.