I built an all singing, all dancing Speedline Tamar last year and can offer one set of options that I know work and I also used the same Escs and motors etc as you.
As already said, for both main motors and the bow thruster it is so much simpler to use 1 battery per Esc. I powered the hull radio receiver from the Mtronics bowthruster Esc which was powered by a 6v, 7Ah SLA; simply because the bowthruster is used much less than the main engines. Each of the 3 motor batteries is independently fused and switched and 20amp fuses are all that is needed. Separate switching allows a managed power-up of the systems and allows you to hear the individual brushless self-tests. The hull lighting obtains power from the receiver power supply, so if the stern and deck lights etc are on then the receiver has power. The transom door is servo controlled directly from the receiver.
The Cabin has a completely separate electrical system - a 7.4v Lipo cell, an in-line BEC and a second radio receiver off the one transmitter, again with it's own fuse and switch. This manages the main mast, radio masts, radar and all cabin and mast lights through a range of R/C switches and relays.
I am not sure your radio has sufficient channels, I use a Turnigy 8 channel system and still had to combine functions, such as main mast, radio masts and main mast lights on the same switch.
I certainly don't want to draw the circuits though.