Yes, not good practice to use the start battery for domestic use. The parallel function is for exactly the opposite, to use the domestic bank to start the engine in the unlikely event of the start battery failing. The development of "leisure" or dual purpose batteries dates from the time when boats commonly did not have dedicated start batteries but two batteries the same used for both engine start and domestics with switching to to give a choice of which one to use.
More modern boats like Bavarias that have a split charging system and dedicated start battery allows the choice of the most appropriate batteries for each purpose. Leisure batteries are still available but are not as good as AGMs for domestic use, nor as good as high output, usually AGMs for starting