Vickers KCG 3 350D Z M U

Five liters per minute puts this valve in the lower flow range, but it fits right in line with what proportional valves in this class are built for. Die casting and foundry work is actually one of the listed application types for this unit, so it's not a mismatch. Whether that flow rate covers your full system demand depends on your circuit design, but for proportional control tasks in that environment, it's a reasonable fit.

Buna-N provides decent resistance to saltwater, so it's often chosen for marine applications. However, prolonged exposure can cause degradation, so you'll want to inspect the seals regularly, especially in demanding offshore conditions.