What size outboard do you have and what is the amp output on the alternator? The MK 2-DC outputs 10amps per bank in on optimal output, this is what determins how well the DC charger will work. Which should be no different then the charging speed of a MK220 onboard charger for a comparison.
Not sure about if it is good to run that charger to your starter because it will now have 2 charging sources and the one will be 10amps…But not positive.
A yamaha F75 alternator puts out 25amps. You may not get the performance you are thinking from a DC charger. I opted to not do the DC charger and added a second battery in parallel with my starter and moved the accessories to that battery and put in a VSR on the positive side of the parallel connection. When big motor is running it supplies 25amps to both battteries and when motor is off the batteries disconnect. I run 2 graphs, SonicHub, livewells, lights, and NMEA2000 network with out ever needing to charge my battery when I get home. Been working great for me and at about 70-80 bucks to rig up it was even better!