Water Faucet Conversion

Has anyone successfully installed household faucets in the galley and/or head where plastic 15 mm piping is installed?
I picked up a couple of faucets with typical 3/8 threaded tail pieces. I was hoping that the tail pieces could be connected to a Whale WX1583 connection using a copper reducer. The tails turn onto the male reducer fine. But there is a slight leak between the reducer and Whale adapter. The threads appear to to be matched and there is an O ring on the Whale connector. I've tried both T tape and thread sealant.
May do 5200 next.

You are absolutely correct. I wish the industry standards were, well... standardized across all industries. The copper fitting is tapered, the marine fitting is not.

In the meantime, I' ve sealed them with 5200 and let them sit over night. All dry now.