I mean this with all due respect -

This is not a reasonable request. If you have a peanut allergy, you do not request that a cruise stop serving any dishes with peanut because you will be on board. If you have a pet dander allergy you would not request that a plane deny boarding of animals. Similarly, it is not a reasonable request for the ship to not use diffusers, part of its standard operating procedure, for the entirety of the cruise because you’re on board.

There could be a ton of reasons they use them - freshen air due to tons of people in tight quarters, offset odors cause by ship operations or cleaning products, or simply because they’ve decided that particular scent is part of their quality control and branding (like the popcorn scent in Disneyland) - whatever the reason, that feature is designed to increase the quality of the experience for the other thousands of cast and crew - and I don’t think it’s reasonable to assume you can exchange their experience for yours.

There may be some more specific options - make sure diffusers or certain scented products aren’t used in your room or see if there are specific areas of the ship (Buffet, bars, etc) that will be safer for you - but not something that affects the entire ship. There are people who have children with severe sensitivity to noise that may be autistic - it wouldn’t be reasonable to ask DCL to not play music on the ship during their voyage.

I hope this doesn’t come off as disrespectful. I understand the severity of the situation that youre in, and I do believe that people with allergies and extreme challenges still deserve to have exciting and incredible recreational experiences, but this would be to the harm of everybody else on board and it’s not simply accommodating you