Marine mammals, perhaps captured - one hopefully has them on board (3,4)
I believe the answer is:
sea legs
'one hopefully has them on board' is the definition.
'marine mammals perhaps captured' is the wordplay.
'marine mammals' becomes 'seals' (I've seen this before).
'perhaps' becomes 'eg' (for example).
'captured' indicates putting letters inside.
'seals' going around 'eg' is 'SEA-LEGS'.
(Other definitions for sea legs that I've seen before include "Molten rock" , "musical" , "Balance on board" , "Ability to maintain one's balance on a ship" , "What a good sailor has" .)