That would require gross negligence on the part of LE and the ME/forensic pathologist.
I doubt this is occurring where there are laws that apply to these circumstances (unattended, suspicious/accidental/suicides/unexpected) and require an investigation/examination by LE/ME/forensic pathologists. I think most people, in general, are aware of this and don't think that dumping a deceased person in water will be called suicide by default.
A body found in water is not presumed to be a drowning and a drowning is not determined to be a suicide without convincing evidence.
If drowning is found to be the cause of death, but there was no witness to the drowning and no evidence of a homicide, it's usually determined that the manner of death is accidental or undetermined, even if some suspect it was suicide lacking strong evidence.
"those involving poisoning or
submersion were most likely to be classified as undetermined."
Injury deaths can be grouped into four general categories: accident, homicide, suicide, and undetermined. The present study investigates the use of the "undetermined" category. External cause of death, as well as demographic and other variables, were abstracted from death certificates of the...
pubmed.ncbi.nlm.nih.gov
Peruse tables with characteristics of deaths requiring investigation or autopsy in each state.
www.cdc.gov