if a person swims in the ocean, is there a way to measure diatoms either on him, in him, his blood, his clothes or his habitat?
For movie, I'm looking to tie victim (homicide/drowning) with murderer, who claims he was not near ocean that day. I know diatoms are found in deceased after drowning, but do the living carry them as well after exposure?
Yes, anyone who swims in the ocean or in a lake will come out with diatoms on them. Diatoms are tiny single-celled organisms with distinctive silica shells. The exact diatom population of a body of water can give forensic scientists a sort of diatom fingerprint.
Diatoms will cling to clothing, particularly to folds and seams. They will also cling to head and body hair, and can get into the ear and nasal canals as well. They can be recovered by swabbing or by a rinse of sterile water.
Taking a shower would wash most, if not all, of the diatoms off of a person, so you want to keep your character out of the shower after the swim, or else have your forensic personnel isolate the diatoms from clothing. I have added a link below about "pollen rain", because the forensic procedures for pollen and diatoms would be very similar.