Oooo, nice and island-y. Or something syncopated. Love it. I like the beat, the energy. The lyric is totally suitable to the summer challenge theme. The rhyme scheme is a little inconsistent, but it doesn't bother me. I just really like this.
I did notice a couple things in the lyrics that didn't feel quite right. "He was sat" (first line) seems like a really odd grammar error. "He was" needs "sitting" to complete the verb. If you want "sat", you shouldn't have "was". The line can easily work out rhythmically if you say "He sat on the beat in the summer".
Second one "But it wasn't them at all". I would expect it to say "But it wasn't her at all". Unless you're trying to be more all-inclusive and not be gender-specific about his summer lover, in which case, never mind.
Although, the same thing crops again in the next verse with "and something changed in their eyes". This one is even more confusing, because you're using "they" and "their" already in the verse, so it makes it feel like that idea is continuing. Except, in context, it can't be. So that one is a problem for me.
If you're trying to leave it open for him to have a summer lover that's either male or female, I would prefer the pronoun references be more clear. I haven't thought of a way to do that, though. Yet.
Anyway, just my opinions throughout, of course, so feel free to ignore me.
Vicki