The newly renovated Shangrila Sentosa Resort Hotel is a shipspotters heaven. Take a room with panorama view on a high floor. I'll be there again end of February.
There are no real shipspotting harbour cruises available in Singapore - at last, I didn't find any. I went to Marina South Pier - a map is included. If you look around there in the Terminal building maybe you find a skipper who is interested to show you the keppel port. I payed 60 Singapore-Dollars for a one hour trip through the keppel port - the boat was lousy, the view excellent. If you look for my fotos at Shipspotting (from Bodolinsky), all the pictures taken in Singapore I made on this trip last year.
Have a nice time and good luck.