There certainly would have been cargo ships, and some of them might have have taken a small number of passengers. Some shipping lines/routes still offer such today, though IIRC I've only ever met one person who made such a journey. See for example:
https://www.cargoshipvoyages.com/WhatToExpect/
Re: passenger boat. Luxury cruise ship I expect, I'm not sure there would have been a market as late as 1980 for a 1600 mile ferry journey... ?
See if you can borrow for pick-up a copy of a Lonely Planet 'South East Asia on a shoestring' from back then. They were publishing revised editions every couple of years. Here is the 1978 edition:
https://www.amazon.co.uk/South-East-Asi ... 0959808078
There was another in 1982... A copy of that book from around then will give you a definitive answer on the SIN<>HKG travel options back then.