Plumbing question- 91' 350 DA

[LastCall350]

Getting ready to add a shut off valve to separate the water system on the back deck of the boat from the inside so i can leave the inside system usable. My 1991 350 DA has the gray plastic (pex?) water line in it. Is this 15mm or 1/2"?

 

[HawkX66]

Check your fittings. They should be marked. Probably 15mm.

 

[Stee6043]

I'd also wager metric. freshwatersystems.com is a great source for the fittings, for the record