Def get travel insurance. My last cruise I got sick, had to see the inboard dr it was $175 for a shot of something to make me stop being sick. Excess on travel insurance was $200 so didn't cover that but if need something more for your MS it would def be exy.
They have hand sanitiser everywhere on board if you can handle the smell use it. Agree eat at the restaurant not buffet, doesn't cost anything so you might as well as the food more freshly served after cooking.
I'm a puker on sea and while pregnant so I have my fingers crossed for you that you'll be ok :/