Keep in mind that "heavier" is not quite the term you're looking for. You should be trimming your SeaPerch to be neutrally buoyant, so unless the new payload creates significantly more drag than your old one, your existing motors should work fine (just with decreased acceleration in response to the increased mass).
The answer to the question of torque vs RPM really depends on the propeller you use -- they should be evenly matched. There is a fair amount of theory on this (for example, Mark Drela's lecture notes from MIT), but you can also just approach it in a practical way.
The basic idea is to work with the power curves of the propeller and motor (both of which are measurable):
(more here)
Based on this, you should be able to work iteratively with a few propeller designs after you pick your motor. There are also a lot of guides for this (like this one, providing the image below) that explain how to choose the propeller based on the power of the motor that you've already selected. You might not be measuring your engine power in units as large as BHP, but the technique is the same.
As always, you'll have to run your own thrust-vs-rpm tests on a variety of propellers to verify your calculations.
