Take a cab. Taking the bus and transferring at DTD will probably take about 90 minutes, possibly more if you're unlucky. A cab will be about 15 minutes and cost less than $20.
Monorail will run till midnight. Buses will run upto 12:30 a.m. that night. I am not sure how late the boats will run.
I do think that the taxi is the best though.
Agree with the suggestions for a cab. We are doing V&A in a couple of weeks and are staying at OKW. Even though we will have a car this trip, we are going to taxi over and back for this occasion. Don't want to have to drive back to OKW after having drunk all that wine at V&A!
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