its actually a lot easier and less stress if you're doing the full course over 4 days on the island. i think you will enjoy it more too.
however, the cons of going up on your own to do that are:
- difficult to follow up if you dont get your certification card, compared to a local dive shop
- won't meet hot babes or hunks from singapore who can be your future dive kakis
- more difficult to go on other trips in future because you don't have a local dive shop that you frequent
- hard to get equipment purchase advise
- you miss out on getting plugged in to the local dive community!
- might potentially be more expensive and troublesome because you have to arrange everything on your own. what if the van doesn't turn up? or the ferry is cancelled? at least with a local dive op, everything will be taken care of. just board the van/bus, and switch off your brains. before you know it you're on the island and diving. i'd gladly pay $50-100 more for the ease and convenience. not to mention having someone to scream at if something goes wrong...