Rising river covering top half of chap's boat (5)
I believe the answer is:
yacht
'boat' is the definition.
(yacht is a type of boat)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for yacht that I've seen before include "Luxury purchase" , "Cathy (anag.)" , "Sailing boat" , "Sailing or motor vessel for cruising or racing" , "Pleasure-boat" .)