I once had a severe ear ache while on a Disney cruise and I went to the medical facility onboard. The doctor diagnosed it as an ear infection and prescribed erythromycin. Total cost for the doctor's visit was $90 and for the prescription was $110. Total amount my onboard account was charged was $200.
I submitted the paid receipts to my health insurance company and they reimbursed me for my expenses.
But $900 for some antihistamine sounds a little extreme.
Then again, if the ship's doctor had to 'consult' with another doctor, what does that tell you about his credentials???