Adding my own thoughts on a marine policing/dive and marine unit I think the simplest way without overcomplicating things to start with would just be a general marine vehicle and a towable boat trailer that acts as both marine policing and inland rescue, covers most services without needing devs to make new buildings and such for the dedicated boats the likes of MPS and MODP have (which could easier be added later down the line)
100% buddy, down here in the met MPU is such a broad thing. General marine policing, dive teams, confined spaces search team. MPU work with MO19 and SO15 to bring a maritime ARV capability. This update is very simple in my eyes and only requires a building expansion and a couple new vehicles.