If you will be getting your Visa in Singapore, make sure to allow for 3 days processing. I just got a Visa there recently and there is (apparently) no such thing as express processing. Be ready for long lines and waiting times, the embassy is quite busy and the staff does like to take its time.

Also, do call ahead and get all the facts about what you need in terms of paperwork and pictures... it varies depending on your visa type. For tourism, make sure you at least have one passport photo.

Bottom line... if you can get it on arrival (I dont know about the Ferry Terminal), it will surely be your best bet.