The drop rates for the breathing devices need to be checked. In 6 months I've gotten a ton of Typhoon Underwater Breathing Device designs and no Seabreeze.
The auction shows pretty much the same thing. There have been 2 seabreeze on auction in the last 6 months (I check daily) and tons of higher value underwater breathing devices.
Could you maybe make drop rates proportional to the scale of the device so the more valuable devices drop less?
I think this applies to all Seabreeze designs. They have an extremely low drop rate for some reason. Kind of strange that tier 2 designs are the bottleneck for the other levels (which are far easier to obtain).