Since I have an engine with a ground I haven't delved very deeply into this issue. If you don't have an engine and don't have any AC on board there is no problem connecting all the DC- to a common bus bar. The problem arises with shore power and a path from the shore power to the DC system as might happen with an inverter/charger or just a charger. The DC- is connected to the engine block for 2 reasons, to serve the DC- for the starter and sometimes the alternator. It also provides an AC ground through the prop shaft to water.
If you intend to connect to shore power and AC devices on board, then this is where you want to get advice from an ABYC certified Marine Electrician.