The resort monorail will be running, but I'm not sure how early the first boat leaves WL (guessing 7ish like the monorail but I can't say for certain). There's a grace period if you make it there a little past 8, and I've found Kona breakfast around that time of morning to be fairly quiet (at least this time last year when they reopened following renos).
But while I love that boat ride, if you'll be spending it worried about whether or not you'll make it on time (waiting for the boat, plus 10-15 minutes dock-to-dock, plus ~15 minutes MK-Poly on the monorail), and you're a small enough party, I'd just take an Uber there. It's so close that'd likely be a <$10+tip, <10 minute affair. Seriously. It's one traffic light and a mile or less by car, versus circling the lagoon on the monorail. Then, after breakfast, you can do any monorail or boat travel you want to.