We haven't tried Homecomin' yet, but have dined at 'Ohana many times, and tried the Boathouse on Monday.
For food and service, the Boathouse wins, hands down. We had steamed clams, a selection of oysters from the raw bar, and some stone crab claws. Our waiter was beyond excellent, especially at describing and recommending which oysters we might like. The cocktails were also well crafted and delicious, and accompanied the fresh seafood perfectly. Atmosphere will vary greatly depending on where you are seated. We had a table on the veranda across from the over-the-water bar, which was relaxed and pleasant with a wonderful view, but a little chilly that night due to the stiff breeze coming off the lagoon. The inside area we passed through was much more bustling, chaotic and loud, but also out of the wind, which could be a advantage depending on the weather. We didn't order entrees from the menu, so I can't speak to the quality or preparation of those. This place is pricey, though, and not inclusive and all you care to eat like 'Ohana, so if you have big eaters that may be a consideration.
'Ohana is, well, 'Ohana! We've dined there more times than I can remember since 1999 (obviously we like it!) and never had a terrible experience. But some times were better than others, the food is decent but not exceptional, and the service is definitely spotty. Advantages are that the atmosphere is fun (but chaotic), servings are plentiful, and there are enough different dishes served that everyone should find something they like, especially the bread pudding. Also, the drinks from Tambu, and Trader Sam's if you stop there, are excellent and very conducive for having fun with friends. And if your group includes any carnivores with big appetites, this may be the place for you!
Both are good choices, you probably can't go wrong with either of these unless someone is having a bad night, which is perhaps more possible with 'Ohana than Boathouse. Good luck and have fun!
we also did Boathouse and I loved it! We're from New England so I was happy to get some seafood and the lobster roll certainly didn't disappoint. My DH got some sort of steak dish and really enjoyed it. It's a nice atmosphere and we were seated right by large french doors so we got a nice breeze off the water and were able to watch the amphicars go in and out. 
