Hi Mark,
I second PerS.
The foam core is unaffected by potential water ingress, and the
water cannot propagate.
The life of teak decks depends on conditions (climate, cleaning,
winter storage under cover, etc.). Our deck was originally 12mm
thick. In Scandinavia it could last 35 years, in the Med perhaps
as little as 15 years, and require occasional re-caulking and
re-bunging.
Regards,
Leo (s/y Firefly, #6)