It is interesting to know that Singapore-Johor Bharu undersea tunnel will be the next mega project in Malaysia and Singapore. Currently a feasibility study had been carried out since November 2011 . Consultants from both countries were appointed for a detailed engineering study. The undersea tunnel will be around 1km which is slightly shorter than the existing Causeway.
The decision of Malaysia and Singapore government to built undersea tunnel instead of continuing the existing plan of crooked bridge is quite a surprise. The cost of Singapore-JB undersea tunnel will be definitely higher, and it will take a lot more time to be completed compared to the earlier plan of crooked bridge.
When I was involved in the PGB-Keppel (Singapore Gas Pipeline Project) back in 2006 , the deepest seabed between two land is approximately 20 - 25m from sea water level. Therefore the underwater tunnel must be built further more from the deepest seabed . Below is an example of an undersea tunnel which had been constructed in between UK and France.