Can you get onychomycosis from shower floors?

Possibly: It is possible to get athlete's foot from shower floors (in hotels, gyms, etc) caused by a dermatophyte which can later cause toenail fungus aka onychomycosis.

Possibly: Toenail fungus grows in dark and moist spaces. You may be exposed to a higher concentration of fungus in a contaminated shower floor than other similar surfaces. However, ultimately it has to do with your susceptibility to the infection.