Consider a Coke can.

When it's closed and pressurized you have a very hard time crushing it. The internal pressure is converted to a force of tension that resists deformation. Once it's been opened, you can crush it with one hand from the side. But it's much stronger along the axis of the cylinder, since the force is directed through all the material rather than deforming it inwards.

A rocket if scaled down to the size of a coke can has walls much thinner than a coke can, and is much longer relative to its width. You can create great torques by hitting the sides to bend it, or crush it inwards. Imagine the force of tens of tons of water suddenly slapping onto the side of this tank as waves lap around, unevenly across multiple parts of the tank.

Consider a rocket.

It must, with the least possible amount of mass, generate a high acceleration along its direction of motion while subtending a very small surface area in that direction of motion. This dictates that it is long and thin, and able to withstand high forces along that long axis. But every kilogram you add to its mass is at least one, sometimes multiple kilograms you can't get to orbit. You make it withstand the forces of its operational environment well. Other forces not so much. As far as I know most rockets get a good deal of their shear-strength perpendicular to the long axis from pressure within their tanks - some cannot even be held sideways without being pressurized. I've seen figures to the effect that more than half of the structural strength of a falcon 9 tank comes from it being pressurized by inert gases during operation.

Optimizing for its operational environment plus mass puts limits on how much you can optimize for other things. There is no one abstract 'durability' factor.

Salt water does not play nice with metals, especially fine tubes and components that will be channeling ridiculous energies and god knows how many RPM and very fine precision. It corrodes, it dries up and gums joints, etc. Shuttle SRBs were a different story - they were big, dumb, thick, wide metal tubes filled with fuel that burned like a sparkler, not fine actuators of active control. If someone could find a simple coating that could protect complex powerful machines from the damage of salt water without tradeoffs, I'm sure the navy would LOVE to know about it.