I once took a lady out to a secluded bay where she could complete her refresher course as she had not dived in a few years. The boat journey there was not particularly long, taking only around 20 minutes. But by the time we arrived she was suffering from extreme seasickness including vomiting. The water wasn’t that choppy but her body had definitely had enough of it and she decided to postpone her dive till the next day. I felt so sorry for her!
Moreover, vomiting causes dehydration. If you have been on the boat on your way to a location, then chances are that you will not have time to rehydrate enough. Dehydration can cause decompression sickness during or after a dive, which is no joke!
However, should you have time to rehydrate, do drink plenty of water and even energy drinks to get yourself ready. At worst, your driving instructor may suggest that you go on a shallower one, one that will be less trying on your body. It’s worth looking at all your options from the onsite dive center before you get in too deep.
You should always keep your diving instructor informed about your current state to allow them to help you and make the safest decision about the dive. There is nothing to be embarressed of – it’s much safer to be honest.