Vessel surfacing? One is designed to carry passengers (3)
I believe the answer is:
bus
'one is designed to carry passengers' is the definition.
(public transport vehicle)
'vessel surfacing?' is the wordplay.
'vessel' becomes 'sub' (submarine is a type of vessel).
'surfacing?' is a reversal indicator (as opposed to capsizing).
'sub' written backwards gives 'BUS'.
(Other definitions for bus that I've seen before include "Public service vehicle" , "Public transport vehicle" , "People carrier" , "Of projectiles; (go) mad (slang)" , "Large passenger vehicle - small business!" .)