Here's the sub - it's going to be in very low gravity underwater, so it uses two low-powered nuclear engines for forward thrust and power, and some electric propellers for steering and attitude control. It's designed for comparatively short missions, a few hours at a time so no galley, bunks, or shower, packed lunches, etc. This may turn out to be a problem for the characters...
The picture has some jaggies but It'll be a PDF based on a vector graphics file so print quality should be as good as the printer supports.
Now for the question - assuming that there's liquid water and ice all the way to Ceres' core, and that basically that's all that Ceres is made of, and that it's diameter is 487 KM, what is the pressure like at different depths? Can I assume that since gravity is 0.028g the pressure at any given depth will be about a thirtieth of that in Earth's oceans? or is that a ridiculous over-simplification? What happens as you get nearer the centre of the asteroid and gravity decreases?
The current depth record for a submarine is around 6500 metres, so what's the equivalent for Ceres? Could you get to the actual centre of the asteroid?
later I'll probably add a lot more clutter - life support machinery etc. - to the plan, it's a bit too spacious.
later still Simplified the exterior - dropped the big engine pods in favour of a single engine that looks a bit different from the spaceship engines I've used in other plans, and put thicker stalks on the steering propeller units - and added a bit of interior clutter, a camera, forward light, etc.