Will this design cause corrosion from a ground loop?
Although some pictures would be needed to determine a corrosion pattern and investigate the cause in deep, we could assume that:


Galvanic corrosion should be completely discarded. Low DC voltages (i.e. 3.3V, 5.0V) with respect to ground (sea water) would not be enough to cause a relevant corrosion thickness (less than 2 nm per month). As per skb.com:



  An external field with 5V maximum voltage and an internal resistance
  of 500 Ω would give a corrosion attack of about 2,600 μm in 100,000
  years.