If you fired a rocket from a boat, you'd likely flip the boat, or knock someone out of the boat which is not likely to be simulated.
While the boats with remote MK19s on them do carry a lot of hardware, they are not unflippable, nor designed to withstand the force of a rocket behind fired from them.
You would never fire a rocket from a vehicle, especially the top of one, all that force would flip the vehicle over if it were in motion & likely kill the guy firing it as he was crushed.
You do not fire rockets from APCs/Tanks simply due to the fact you are a big target, it's a confined space inside, in a tank you barely have room to squeeze yourself through. What's the point of a rocket when the vehicle you're in fires 30MM rounds or 120MM which do more damage, are superior in accuracy/range to a rocket equipped with stabilizers.
While the cS does not produce backblast, it still produces force, you need a solid firing position to prevent the rocket from hitting the ground in front of you or doing something equally absurd.
The CS variant of the AT4 is also more costly than a standard AT4, they're also single use, thus making them not so worth the cost compared to re-usable shoulder mounted rocket launcher systems which are tried/proven with the same capability, sometimes better.