For a topic sentence, or the beginning sentence that signifies the main idea of the paragraph, you could focus on the fact that there are a variety of things to do at the beach (perhaps the 3rd sentence). To create a hook, or an eye-catching sentence, you can use the childhood memory of learning to swim, but perhaps in more detail. In other words, recreate the scene rather than just saying it was a happy memory (i.e. I'm five years old and am amazed by how weightless I feel in the water). Once you create the hook and topic sentence, you can talk about other activities at the beach as your supporting evidence.
Say this:
Me and (state the name) were due to reach the common goal of getting full marks on our Language Arts quiz. But, we had a disagreement on what the right answer for question (random) is.
I hope this helps you.
