Longer boat trips to the island is also possible.Click here to check their timetable to make sure you have the current info. If you're willing to get up at the crack of dawn, the Chao Phraya "green flag" express boat offers a direct service from BTS Saphan Taksin to Pak Kret (pier N33), but during peak hours only. Boats operate every 15-20 min in both directions from 06:15 to 08:00 in the morning, and again from 15:30 to 18:00 in the evening, with no service on Sundays. The trip costs 20 baht and takes just over an hour. The service from Pak Kret operates INTO town in the morning and OUT of town in the evening.
Outside peak hours, the closest you can get is Nonthaburi pier, the last stop (pier N30) of the normal Chao Phraya Express Boat. From here options are:
Take the air-con van service (just 10 baht) or public bus 32 to Pak Kret, then head to the ferry pier serving the island. There are no signs in English, and so finding your way can be tricky.
Hire a river taxi, for which touts will quote prices around 500 baht. With enough haggling this may be a reasonable option for a group.
If you can't get a return trip for less than 200 baht (and you probably can't), it'll be cheaper to take a taxi to the temple of Wat Sanam Neua (80-90 baht) in the neighbouring district of Pak Kret, from where ferries shuttle across the river to the island pretty much non-stop for the princely sum of 2 baht (return). Just tell the cabbie "Ko Kret", they will understand.