The first thing to note is that it is generally a good idea to carry your passport or ID card with you, as there can be occasions where it will be asked for, usually due to security purposes.
When this is done, always check whether you need clearance to either visit or stay. For some islands like Pulau Semakau, you will need clearance from the National Environment Agency (NEA), which can take close to a month, while others may require permits to stay overnight. In those cases where you are not allowed to stay, like Kusu Island, make sure to have a return ride planned in advance.
On the other hand, if all you want is to explore the Singapore River and see the historic centre through a key route, just check along the outside of the Esplanade. There, you’ll find a number of booths for you to book a bumboat to wade downstream and back, with different boats offering different services.
When it comes down to it, a very important element is cost. Some islands will require you to charter a boat specially, like Sisters Island and Pulau Hantu, which usually means negotiating and haggling to get a decent deal. The others have a whole set of timings you can plan a trip around. One thing to keep in mind though is that on weekdays, there tends to be fewer ferry services. So make sure to call or check online before heading out. Most of the ferries are fairly affordable, generally from $18 upwards. In some cases, like Pulau Semakau and those that require a private charter, the entire trip can cost you up to $400 just to get a ride.
There are a number of piers you can book from and again, it all depends on the island. Marina South Pier is the most popular as it has one of the largest number of ferries heading to the southern islands. For Pulau Ubin, you need to head to Changi Point Ferry Terminal to grab a bumboat and they usually leave when it’s full or if you buy out the whole space. For some of the more westerly placed islands, your best bet is West Coast Pier.
A final point to consider is that these ferries and bumboats come in all types of shapes and some are less comfortable than others.
The ones that head to places closer to the western part of Singapore, like Pulau Hantu, can be less comfortable as most of the islands are used for factories. Hence the boats that go there tend to be service type boats, which can be down to their bare bones.