So the laser is running making some 2.6 ROV internal structures and there's a geiger counter sitting on Erics desk. I decided to see about wiring it up, check out the software, etc. It looks like it just fits, but obstructs the camera movement without a custom USB cable. The headers on the OpenROV Controller board have tons of IO and it gives you power and ground all over the place too. So using one of the 3 pin headers that's an extra part in the 2.5 kit you can wire it up to 5v, GND, and D14 . If you look at the schematic available in PDF form here on page 1 you will see that D14 is not used for ROV functions. On page 5 it looks like D14 turns into PJ1 on the ATMega2560 and supports an interrupt. This pin is also UART3 it looks like so that is a bit of a waste just to get a pulse from a geiger counter...but that's OK!
So the first step is good to go, it fits and it powers up, now I just have to get to learn the art of adding geiger.cpp and geiger.h in to the loop next to light.cpp etc!