Alex, 

Amel used PEX Plumbing on 54s. PEX is used in Europe and the US. I am not sure if the sizes are the same, but I think so. You can buy PEX in the US anywhere plumbing is sold. That white this was added by someone. Maybe the gray PEX tube was glued or epoxied inside. Let me know what you find. 

Best,

CW Bill Rouse
Amel Yacht Owners School - www.AmelYachtOwnersSchool.com
720 Winnie St
Galveston Island, TX 77550
+1(832) 380-4970