O, firstly, check that your verticals are vertical...the Merlion and the buildings after it are slanted to the right.
I cannot remember if SXIS cam has histogram, but, when you shot with Manual mode, you can see the exposure meter guiding you to expose more or less....however, this is assuming that you are metering with the right mode at the right spot of the pic....if you are not metering the right thing, the exposure will still be out.....
Yah, try to shoot with low ISO...personally I don't like going higher than ISO 400 when I was using S3IS. That's why tripod is so so important. And yah, the external Hot shoe is are really good improvement on the S5IS that makes me drool...