Pusser hit it on the head. "Ready to go". Yes, techically, coming over the brow at 0749:59 you're not adrift, but you're not ready to work. The phrase "leave expires onboard at 0750" implies that the working day begins then. Engineering watch being told to be onboard for 0530 flashup means, to me, onboard, in uniform, not in the heads taking a dump, not in the cave getting coffee, but in the MCR, ready to go.
OP, you don't mention whether you were on time in the sense of walking through the door on the dot, or in the sense of being ready in all respects, so I'll assume you were. But if you weren't, that's probably why your superior ripped you a new one, and I'd agree with him.