There are various options
From Marina South Pier, you can get a short trip to the Western anchorage on a tourist boat "Cheng Ho III"
Same place you can get a ferry to Kusu Island which is a fantastic spot for passing traffic.
(Beware thare are no facilities on the island so you must take any food/water.) Also last boat back was about 16:00 back in 2013 DO NOT MISS!!
Other people (not me) have hired a boat from Marina South Pier to take them around the Eastern Anchorage.
There are views from the Sentosa Island cable car and there are ferries from around Keppel that may be good (not tried)
There are good views from the ECP (East Coast Parkway) but you will need a telephoto lens or telescope as the anchorage is quite a long distance