Mercury has to blame it on something....
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I think the key to this argument is the application of the battery.
I think most V20s are run during daylight and have minimal draw on the battery for short periods before running and thus recharging.In this application, a starting battery will be fine.
I used to drift or anchor for hours, often at night. I used to turn on my floods and underwater light at the dock and fish for stripers under the dock. It wouldn't get charged until the next time I ran it. I used a single 27DP for 18 years and it worked great.
As for a DC, they are for House batteries on large boats with multiple banks or for electric trolling motors.
Here is West Marine's take on battery selection.
Starting batteries
Starting batteries, which crank the starter of your boat's engine, are the sprinters of your electrical system. They deliver between 75 and 400 amperes for 5-15 seconds, and then are recharged in short order by your engine's alternator. Like all lead-acid batteries, they are constructed with alternating layers of negative and positive plates with insulation between them.
Starting batteries have thinner and more numerous plates, providing extra surface area to generate high amperage bursts of current. The two drawbacks of this construction are that the plates are relatively fragile in high-impact environments, and that starting batteries do not tolerate deep discharges, which reduce their operating lifespan.
Deep cycle batteries
Your boat's house battery bank uses deep cycle batteries, the marathon runners of the storage system. They power the electrical loads on your boat when no charge source (shore power charger, engine alternator, wind generator or solar panel) is available. Consider them a kind of savings account into which energy is deposited or withdrawn.
Compared to starting batteries, which deliver high bursts of energy for short periods, deep cycle batteries recover fully after being heavily discharged over longer periods because their design features thicker plates with a high content of antimony. Overnight, their use might deplete 50-70 percent of the battery capacity, depending on the house loads of the boat. When the batteries are recharged, energy is re-deposited into the bank, and the process, or cycle, starts over. Generally, deep cycle batteries should be sized to store three to four times the expected amount of energy to be used between recharge cycles.
Dual-purpose batteries
We generally advise that you choose either a deep cycle or starting battery for best performance and battery life, but dual-purpose batteries work well in some applications. With large, thick plates containing more antimony than starting batteries and an active lead paste chemistry, dual-purpose batteries are a good compromise, tolerating deep discharges that would ruin a typical starting battery. Since they have lower storage capacity than comparably-sized deep cycles, we recommend them for the following applications:
Runabouts or other small powerboats using a single battery for both starting and running loads with the engine turned off.
Sailboats with two identical batteries used interchangeably for starting and house electrical loads.
Boats with one battery bank that does double-duty for house applications and engine starting. Dual-purpose batteries will last longer and give more reliable service than a starting battery, for about $20 more per battery.