Fella at Swansea had to do this 2 years back in an Arrow.
He chose to keep the legs up and made a safe ditching onto very calm waters; picked up by a fishing boat very quickly and the aircraft went down shortly afterwards - still down there! Here's the AAIB report.
http://www.aaib.gov.uk/cms_resources...ANG%208-05.pdf
I also recall a C-152 going into Tampa Bay back in '94 on the approach into Albert Whitted. The moment the wheels touched the water they just dug in and flipped the a/c over on its back. Both pilots got out safely.
As has been already suggested, reference should probably be made to the POH but as a generalisation I would have thought that leaving the u/c retracted would be the correct technique when landing on water or dense, deep or tough undergrowth - there may be less chance of flipping over.