Hello

I do extended cruising so my battery needs are much different.
You can use any type, 12 volt battery for the way you use the boat.

Just as a precaution, I suggest you purchase a portable jump starter. Keep it charged!  This could be an important backup if, for some reason, your 12 volt battery was drained so low it couldn't start your engine.
 

Bruce, please take a moment to create a signature (using mine as a model, maybe) so that those online here can offer more specific assistance. Knowing a little more about you and your boat helps us to help you.

There are aspects to your battery question that haven't been touched on. 

1. Most of our Mk II boats indeed do have two 4D batteries. Perhaps they came from the factory as mine did, with each 4D separate, one connected as 'Battery #1' and the other as 'Battery #2.' Over the years, though, many of our owners have *paralleled* the two 4Ds so as to double the amount of house bank capacity. If each 4D has a nominal capacity of, say, 200 amp/hours, then two 4Ds paralleled would act as a 400 amp/hour house bank battery. Are your 4Ds paralleled??? 

2. If the 4Ds are paralleled, then almost everyone will have installed a third smaller 'automotive-sized' battery for use as a dedicated back-up starting battery in case the 2 x 4Ds house bank ever gets discharged below the level at which it could start the engine. In 15 years I have never used my back-up starting battery, but it's there if I need it. Does your boat have the third battery for back-up engine starting???

3. I hear you say that you would never cruise anywhere that you would need lots of battery capacity, but two things: A) Although the house bank (paralleled 4Ds) may have 400 amp/hour capacity as shown on the label, the amount you can actually access without damaging the batteries by running the voltage too low - VERY rough guess judging by my own experience - is about 100 amp/hours. And B) Let's say you're a day sailor only, no cruising and no anchoring out: Will you ever use the autopilot? (That's a huge current suck.) Or the chartplotter, radios and radar? (More power suck.) Almost EVERY owner wishes they had more battery capacity than they do. Don't underestimate your own need for amp/hours.

3 years ago I found an article by MaineSail about some 12V golf cart batteries at Sams.  They were labeled as Duracell but made by Eastern Penn or Deka.  Either way - quality batteries. They are 12V at 155 Amp hours.  My MK1 only had room for 2 - GC12s without cutting and fitting so I got those and have been very happy with them .  With the 310 amp hours I get about 1.5 days before I hit the 50% mark when running the Frig pretty hard. They were an easy fit under the Saloon game table seat but I also installed a dedicated marine starting battery in a box under the Nav station. Also very happy with that setup as it ensures I don't run short of starting power.  So if you have room I would suggest that too. The height of these batteries could be an issue but I would think they would be a much smaller footprint than your 2 x D cells. You would just have to find a good way to secure them.
I searched Sams tonight and found the GC12s at $199, but also the GC2s at $84.
4 of the GC2s would give you 430 amp hours (20ah rate). The GC2s would be easier to handle than the GC12s  but have a larger overall footprint.
On the charger note, I do have a 40amp charger so charging with shore power or generator is relatively quick. Your 10amp charger would take > 20 hours to replenish 200 ah of capacity.  Something to keep in mind if you're traveling and spending time on the hook. Running the Diesel for 5-6 hours gets old Quick!
 

The suggestions above are all solid. Paralleling the factory installed 4D's until they die is also a good option. Replacing the factory flooded 4D's with replacement flooded 4D's is not the best choice...

Before you buy "deep cycle" batteries there are a myriad of choices and lots more to consider than just how many Ah's per $$. The article below may help you sort out the options.

What is a Deep Cycle Battery?