I fully share most people's dislike of surprise parties. What I do not understand is that the people organising the party do not know this, as they are obviously family members close to you.
I think you have two choices,1, as has been mentioned is to be upfront with the organisers. Say you have heard about the party and heard about it from someone who did not realise it was a surprise (whatever you do , do not name them) and then explain how much you appreciate their plans but you are not a party person and would not enjoy an event like this, and possibly suggest something you would like, a weekend in a hotel with immediate family or similar.
The other option is to start planning an alternative event for about the same date - a weekend away as above, or both going on a cruise, during the discussion, you can refer to your not enjoying parties and perhaps reference a friend whose family did something like that, how she had hated it and still felt guilty because everyone had put so much effort into something that she really didn't enjoy.