The Design of the OpenROV is running 8 C batteries. Since we know it is running 12v we know the C's are running in Series.
Thus if we look at Wikipedia: http://en.wikipedia.org/wiki/C_battery
"The voltage and capacity of a 'C'-size battery depends on the battery chemistry and discharge conditions. Alkaline 'C' batteries can hold up to 8,000 mAh, rechargeable NiMH 'C' batteries can hold up to 6,000 mAh, and zinc-carbon 'C' batteries usually hold up to 3,800 mAh."
We can figure that powered by C batteries we can expect 8 hours of 1amp draw or 6 hours of 1amp draw depending on if the batteries used are ALkaline or NiMH rechargeable.
You are asking about switching to D sized 1.2v batteries. As you already know since your planning on running 10 of them instead of 8. Since you also will be running these in series you already know your mAh @ 10,000. Your looking at replacing a 6K-8K mAh battery pack with a 10K mAh battery pack so you should have more than enough power. You will need to design your own battery packs since the D's won't fit the the space of the design (C's) and they are going to weight enough more that you will likely need more balast.
This is just IMHO on paper since I don't have a kit and am just going by the info on this site.
Also, based on your motor inquiry post I believe your planning to run brushless motors, not brushed.