You need to find out what is included in the price of a yacht charter in Tenerife. If you have allergies and you are not offered a meal replacement, take care of your own individual food and drinks.
To eliminate problems with seasickness, purchase a remedy (for example, Dramina) at the pharmacy.
Don't forget to bring a hat, sun glases and sun cream. Find out if towels and snorkeling gear are included in the price.
And finally, do not forget the camera and a good mood.