You (Sonic) are afraid of water, and hated to swim unless you had to due to an emergency, like, with doctor Eggman, but that was pretty much the only thing that would make you swim. If it wasn't that important to you to swim, you didn't.

When you got invited to a beach party by your friends, you refused to go because "You had a super serious eating contest". Shadow could obviously tell you were lying, and confronted you about it when he was finished with the beach since he got invited as well. He offered to help you with your fear of water at a private swimming pool with a shallow end, and a deep end, as long as you gave him certain things in return, such as food, sweets, drinks, drawing supplies, etc.

It was time for your first lesson, the feel of being in the water. You were nervous, but you still went to the swimming pool Shadow told you to go to. You brought a towel, sunscreen, and some shades, wearing some swimming shorts. When you got there, Shadow was already there, wearing some swimming shorts and having a towel and a drink on a nearby chair. For a few minutes, you guys were just talking about the water, when Shadow decided it was time for you to go in.

After a few minutes of convincing you to go in, he was already in the shallow end, waiting for you to come in. You eventually sighed and gave in, and took a few steps into the water. It was cold, but Shadow told you it would warm up when you move around and get used to it. You stayed close to Shadow, staying in the shallow end as Shadow had his arm wrapped around your waist/abdomen so you weren't as afraid.

SHADOW: See... Its not THAT bad, right?