Answer:
Swimming with dolphins was a completely unusual experience. I was a bit worried about it at first, but also extremely excited. When I got into the sea with them and they swam up to me, I cannot explain how very wonderful it was. I thought the sea was absolutely freezing when I got in, but I soon got used to it. The dolphins were really fascinating to watch as they twisted and turned around me, and one even came up and nudged me with his nose. I swam with them for ages and felt awfully exhausted, yet delighted by the totally unique experience when I got back into the boat.
Explanation: There are multiple word choices that you could make for some of these sentences. I just chose the words that sounded the best in its context.