A spirit level is the best way to ensure that it is level. That said, you can have the horizontal level down pat but your horizon might not look straight. This is owning to curvature introduced in the photograph due to barrel distortion from lenses. PT Lens or Photoshop CS 5 would be able to correct this part, but the horizon would still look wonky owning to vertical perspective distortion, etc.
To add on to the list of problems with getting the horizon straight, the opposite bank in Singapore is almost always very near, even if you are at the beach. Definitely when you are at one of our reservoirs. If you do not align your camera back parallel to the bank (which is close enough, and usually there's more than one, so you would not be able to do it for all), there will be horizontal perspective distortion. Your camera might not be tilted, but because the bank is coming towards you or away from you, it would appear to be slanted.
My suggestion is to make sure that you are well aware of all these things that can make the horizon look wonky, and the steps taken to prevent them, and if they can't be helped, the steps taken to correct them in post processing.
Quick summary: (problems - prevention - correction)
1) Camera tilt - horizontal spirit level - rotate tool, ruler tool, equivalent
2) Horizontal perspective distortion - ensuring that camera back is parallel to opposite bank - Lens correction tool, liquify
3) Barrel distortion - NA - PT Lens, Photoshop CS 5, equivalent plugins, liquify
4) Optical illusion from vertical perspective distortion - make sure upright with vertical spirit level - Lens correction tool, liquify
Cheers.