600W of Solar would certainly put a good amount of juice back into your batteries - depending on the day ( bright & sunny) but remember that flat on the roof of a boat the efficiency will be pretty low …… & even with 600W you might only average 10amps / hr which from my experiences with spotlocking might be adequate on the best of conditions - but will still see your capacity going backwards …….. but hey at least you will be putting power back in .
As for the need for a house battery (dual system) it really depends on what you run …….. back in the day you might have had a sounder & a radio …….. the radio would be used infrequently & so you get away with a marine battery . . How often were these batteries replaced though?
Today ( looking at my boats) I run 2 (500W) sounders , live bait tank , deck wash , VHF & Stereo & much more lighting (low draw LEDs). …….. Now I have been caught out fishing on a dam - where over the course of 3 or 4 days I ended up with a flat battery (running a single Optima D31M …….. with 2 sounders going & only a short run to my fishing - I wasn't putting enough charge back into the battery compared to what my sounders were drawing down and ended up flat ………. eventually I went with a dual battery system with dedicated house battery managed via a VSR . Problem solved.
I've got a mate who has 3 sounders running plus uses a pie oven ….. along with the other stuff - As you could imagine his draw down would be significant ( He does run the motor when the pasta is warming up )
Chris