Complex sentence: In chapter 438, Aokiji is onboard Garp's ship, mostly because he is to see that Garp follows Sengoku's orders, although he doesn't seem to care much, taking a nap on a sunbed.
Split into simpler sentences: In chapter 438, Aokiji was onboard Garp's ship because he'd to see to it that Garp follows Sengoku's orders. However, he didn't appear to care much, as he was seen taking a nap on a sunbed.