Vessel that was submersible but buoyant at regular intervals (1,4)
I believe the answer is:
u-boat
'vessel that was submersible' is the definition.
'but buoyant at regular intervals' is the wordplay.
'at regular intervals' means one should take alternating letters.
'but'+'buoyant'='butbuoyant'
The alternating letters of 'butbuoyant' are 'U-BOAT'.
(Other definitions for u-boat that I've seen before include "Take pool exercise" , "German submarine of WW2" , "Submersible warship armed with torpedoes" , "About (anag)" , "German World War One or World War Two submarine" .)