Being a uboat the real issue is periscope depth and the occasional run deep. That would mean no deeper than a swimming pool, or 2 meters. Any deeper and it all gets a bit silly.
Johnredearth, you mentioned that you were using the MPXV7007G. According to the datasheet, this detects pressures in the range of -7 to 7 kPa, which doesn't make sense to me, since 1 bar (atmospheric pressure) is 100kPa. What's going on here? What is the actual maximum depth you can detect?