This strikes me as wrong in some fundamental ways. I don't know how detailed you want your build system to be, but I'll offer some counter suggestions for doing aquatic type robots.
As above, most robots are not built with water in mind. Immersion in water (especially salt water) can cause problems.
You can waterproof a robot, which makes it resistant to the effects of immersion or soaking in water. This make the robot capable of operating in up to 5 meters of water without problems, though beyond that the problems ensue. I'm assuming that's part of what aquatic covers.
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If the robot needs to have an internal space, the depth it can go is limited by the Armor value. Since each 10m of depth adds 1 atmosphere of pressure, I'd limit depth to 10m times armor value. In theory you can armor the compartment, but leave the rest full of fluid.
The real problem with falling below crush depth is that the failure is frequently catastrophic. Once the robot (or submarine) falls below crush depth and the weak point fails, the entire thing implodes rather quickly. I'd be tempted to start with 3 points of damage (i.e. one roll on the damage table) per 10m depth at the time of failure.
For an extra detail, if the robot is in combat, they "loose" an effective armor rating based upon their depth. For example, if a robot with armor 5 is 20 meters under water, they have an effect armor of 3 (the 2 lost points are resisting the water pressure).
