I understand that but, is the load on a 20 hp diesel that you can start by
hand high enough to significantly reduce the life of the battery?

It depends on what you're using your boat for. If you're starting &
stopping your engine frequently, then maybe a starter battery makes sense.
But while cruising and using my 3GM30 (actually 27 hp) all I used were deep
cycles and they worked fine.

Another topic, one maybe you've thought about already, is how well the
alternator on your Yanmar is charging your batteries. If your charging
system isn't up to snuff, doesn't matter what batteries you're using, you'll
never get full cranking amps out of your batteries.

Paul