Iceberg (Dirk Pitt #3) by Clive Cussler

A routine survey mission over the North Atlantic exposes a missing luxury yacht frozen inside a million-ton mass of ice.

The ship had vanished en route to a secret White House rendezvous, making it the responsibility of the National Underwater and Marine Agency to find out what happened.

In other words, it's time for Dirk Pitt to cut his sunny, California vacation short and get back to work.