The keyword in this flat (HAT) can be replaced by two different words which are anagrams.

We sail the brine on HAT,
Aboard a unit that
The Navy brass has said
Can be deployed pre-Wed.

HINT:
What day is (immediately) pre-Wed?

Answer:

We sail the brine on SEA DUTY,
Aboard a unit that
The Navy brass has said
Can be deployed TUESDAY

Credit:
The Enigma 
January, 2020
UCAOIMHU