Actually, it's not the shocks themselves that have collapsed, but either the springs or possibly one of the suspension components have failed. However, the Altima (like many Japanese and European cars) uses what's called a "McPherson Strut" which is a combined spring and shock absorber unit.
As long as you have not exceeded the GVWR (Gross Vehicle Weight Rating) of your Altima, there is no reason to blame the carrying of scuba tanks for the problem you are having. As with most modern cars, your Altima was equipped with 3 seatbelts in the back, meaning Nissan meant the car to carry 3 average sized adults in the back...though they don't pretend it's going to be a comfortable ride :11: You'd need quite a few scuba tanks to equal that much weight.
BTW what year is your Altima? Reason I ask is because of your location(s) and the harsh winters, which means road salt. If it's an older car, it wouldn't be unusual for suspension mounting points to rust through, particularly the rear strut mountings (when you open the trunk, it's the long cylndrical shaped bulges above the wheel arches). Also, if NYC's roads are as bad as people say, the constant pounding of potholes could eventually cause suspension failures, especially if already weakened by road salt.
