Some background: I live in a state in the coast of Mexico with my mother, my cousin (lets call him Pete) and my grand mother. Pete's family (his mother and 2 sisters) lives in another state and came to visit during Christmas/New Year's eve and they'll be with us for a few more days.
I will try to be as clear as possible with the situation.
This weekend during new year's eve we had some more relatives joining us to dinner and celebration, everything nice. Then they came with the idea that we all should go to the beach next morning, Pete's family said yes, I said multiple times I didn't want to go, so did Pete and my mother, but the rest were very insistent saying that all of us had to go to be together and that sort of stuff. In the end they accepted that we were not going to go with the excuse that somebody had to stay home to look after our 93 years old grand mother (so far so good).
Next morning our grand mother said that she would go too (I don't know if she came with the idea or my family did, it doesn't matter), sooooo the problem appeared again. I was woken up at about 9:00 am to go to the beach with them even when we went to sleep until 3 in the morning or so and I clearly said I did not want to go. I stayed in my bed until 11 hoping that they would go without me, but my mother came and said grandma and her were going to go too and that we should (had to) go with them, at this point Pete still didn't want to go.
Pete and I tried to convince them that we wanted to stay home, I stated that I don't like (almost hate) the beach because I always return full of sand and sunburns if I get into the water. We both stated multiple times that we preferred to stay home and do something here (rest, watch TV, play videogames, read a book, any other thing), I even said that I wanted to rest because I had to work the next day because I was not on vacation, but they were too insistent, they wouldn't leave without us, in the end we accepted.
When they were ready to go (everybody was on the truck we'd use) I tried a last effort to stay and said I was going to the bathroom and I waited there hoping they would run out of patience and leave without me (I think this was rude but I was desperate), it didn't happen, they came and knocked the door and asked me to hurry up. At some point my mother suggested to leave me and go without me but somebody said that if they did then all the time they waited would be in vain, this made me feel guilty so I finally got out and took a book with me to the beach.
Later when we arrived to the beach they all went to swim and the ones who didn't want to go and my grand mother stayed at the table eating, this bothered me because they were so insistent to be "all together" but they left us anyway. We played some boardgames when they came back while they waited for their meal.
The trip to the beach was OK but the return trip was HORRIBLE, 3 more people (a cousin's relatives) returned with us, so the truck was completely full and we had so little space, not to mention that it took so much longer because an accident happened in the highway and the traffic slowed down a lot.
In the end they asked "you were glad you went with us, didn't you? you enjoyed your meal", which I answered "not so much, the food was good but I could have eaten the same in a restaurant in downtown any time".
I would like to know what approach I should take ( or could have) if something like this happens again. I have more family in other states and when they visit they ALWAYS want to to go to the beach and spend the whole day there.
What do I do if they keep insisting? I wouldn't like to lock myself in my room and ignore them because I'd feel like an as*hole, and if they don't go because of that they'd surely blame me.
Edit for bonus
I'd like some advice to make my family understand and accept that I do not want to go with them and that we DO spend time together when they come, we had a nice dinner in christmas and new year's eve, we also went to the cinemas and ate an ice cream.