Hi Rob,
Remember that there will be a voltage drop across the diodes probably in the range 0.5V to 1.5V. If you have devices drawing significant currents - e.g. Tiller Pilot, Ballast Pump - it might pay to connect them directly to one of the batteries, particularly if they are voltage critical (the Tiller Pilot needs good voltage to perform at full power).
When I first got Seatern I had two batteries, not in parallel but each powering different sets of devices (with plugs allowing the roles of the batteries to be swapped or combined if necessary). Now I have only one battery but still maintain separate supplies to some devices. Recently I've realised that the ballast pump should have its own independent supply because it is most likely to blow a fuse, e.g. if it freezes up or the impeller jams. Even though the ballast pump (and everything else) is fused and switched at the switch board, that didn't stop a main fuse at the battery being blown first, probably due to simultaneous demands from the Tiller Pilot and the Ballast Pump. (More details of the evolution of the wiring on Seatern are described in a 30th October 2014 entry in my blog).
Peter