Use of either steel or plastic bar chairs is generally avoided in reinforced concrete marine structures.  Plastic is generally difficult to bond to, due to its low surface energy.  Plastic bar chairs could therefore introduce a path for salt water to enter the concrete, along the interface between the concrete and plastic. 

Marine concrete mixes often exhibit poor workability, as a by-product of specifications that call for a low water/cement ratio and high compressive strength.  This may result in incomplete filling of the voids within plastic bar chairs (refer to photo).

Custom-made concrete spacer blocks are preferred (refer to photo).  The concrete spacers can be custom-made to the correct height with a matching grade of concrete, producing an ideal outcome.