Once you get past the Britishisms "chap" and "larking," the poem seems very easy to understand. It's short, in line length and word length. In fact, it's mostly monosyllabic, with only a handful of longer words to spice things up. And the last stanza is nearly a repeat of the first.
Lurking beneath the waters of simplicity, though, are a few dangerous sharks of toughness. The poem's playfulness with meter and line length might throw you for a loop. Then you have to work out who's talking when. Finally, there's that pesky question of whether anyone really died.
Still, once you've grappled with the different tones of the speakers and decided whether there's an actual or metaphorical drowning, the poem's focus on isolation comes through clearly. But is anybody listening?