Hey Jay- sounds like a cool project! Building a capable ROV from scratch can be fairly challenging and time consuming, but you've come to the right place- this community is all about sharing designs that have already been developed so people can build on each-other's ideas without having to reinvent the wheel.
If you look through the design of OpenROV, you can pick and choose aspects you like and use those for your project, but re-engineer the parts you want to do differently. We're completely open source, so any information you need about the design (CAD files, bill of materials, building instructions, etc) is available. If you come up with some cool ideas of your own, please give back by making them open source too!
As for your particular challenge (flying very close to the hull of a ship) I might recommend a devise that crawls along the hull by using wheels to move and sucks water out from below the vehicle to to adhere it to the hull. SeaBotix uses something like this in several of their vehicles and it seems like a pretty cool idea!
If you just want to fly an ROV close to the ship without actually adhering to it, that's also possible. We've been working on an inner-loop control system that would use a compass to help keep an ROV oriented the right direction and hold the right depth. Feasibly, you could also develop some sort of a system that detects how far the ROV is from a surface (such as a hull) and maintains that distance automatically.
There are lots of possibilities here. Bottom line, I think it can be done, and I think you can get a lot of the help you'll need from OpenROV! I recommend choosing a particular part of the challenge to focus on as what you do differently, and use already developed designs for everything else.
I hope we can help. Good luck and keep us posted!