Here is Pulau Ubin on Googlemaps.

How to get there by bus
Bus numbers 2, 29, 59, 109 will take you there. Stop at the terminus. Head towards Changi Point Ferry Terminal (click for interactive map) northeast on Lorong Bekukong about 100 meters away.
Getting there by Taxi/Car
Ask for Changi Village, turn right into Lorong Bekukong next to the Bus Terminus. The carpark in front of Changi point jetty is often full so familiarize yourself with carparks nearby. Driving directions from Loyang Ave.
Make sure you mention the correct ferry terminal by repeating “Changi Village, Changi Point Ferry Terminal” as there are 3 ferry terminals nearby including Changi Ferry Terminal (3.9km away), Tanah Merah Ferry Terminal (12 km ), Saf Ferry Terminal (2.1 km) .
Ferry to Pulau Ubin island.
Take the bumboat ferry, the boat trip takes about 15 to 20 minutes covering a distance of about 2.4km. It costs $2.50 per passenger and $2.00 per bicycle. Free ride for babies in arms. The boat departs when there are 12 passengers or when the boat get chartered for $30 when there are not enough passengers. Restrictions apply for pet dogs.
The first ferry usually leave at 7-8 am, its usually filled with passengers going to work in Ubin such as contractors or shop owners. No problems if you plan to go earlier as some boatmen, especially older ones, prefer to sleep in their boats. They are usually at the jetty from 2am onwards.
There is no last ferry service. The boatmen stop ferrying when the crowds thin down. On weekends, the crowds thin out from 4pm onwards going to Ubin and from 7.30pm returning from Ubin so expect to pay more by sharing and chartering if you are impatient. You may be stranded in Ubin if the boatmen has gone home so don’t hang around too late.
Update Apr 16, 2010: Visitors’ level seems to have dropped. Its harder to get a boat to Ubin if you are planning to go there for fishing or camping at night. We played it safe by leaving for Ubin no later than 8pm when the boatmen are still around at Changi Point Jetty and sharing the full costs of $30.00 for the boat ride.
We would also call ahead and inform the van drivers to expect our arrival, saving us the long trudge to the interior in the dark. You can call Mr. Moh at +65-9731-7629/ +659084-8827 or Tien Song at +65-93468017.
Taking a taxi home from Changi Point.
The best place to hail a cab is probably at the Changi Point Ferry terminal itself.
You can also book a cab via sms by sending “book 499172 #changi point ferry terminal” to 71222.
Other info
Maps of Ubin. Brochures of Pulau Ubin with maps are no longer given out. They ended as rubbish all over the island. Instead, informative signposts and wall maps are found at all junctions. The bicycle rental shops will provide photocopies if they are needed for route planning purposes. More links on detailed maps.

For info on hotel stay and restaurants at Pulau Ubin and surrounding Changi area.
Bumboat ride video link 1, video link 2