I've done both MK and Epcot cruises...
It can be cold on the water, so bring a jacket. Also, children have to wear life preservers, so have them wear a comfy shirt so perserver can go over, and a sweat top.
That being said, Ohana can be really slow, and check in can take forever (30 minutes), so I think you won't have enough time.... Can you move the dinning reservation up to 5:45 - 6 p.m., or after cruise, or do it another night.
The 8 p.m. time is to check in, pay for the boat, etc, so you won't really leave until 8:30 a.m..... but if you can get there early, and get your captain ready, then you will have more time to leasurely have a boat ride and enjoy the scenary before the show starts.... If your late, they just race the boat into position, either way it's the same price, so I say relax and enjoy.
Boat is expensive, and fits 10 people, so you may want to post on this board and see if anyone wants to share and pay half. Also, captains expect tips, like $20ish???
It's nice. Some of the newer / bigger boats can also play the music while the FW goes on (ask you captain and you may be lucky).
The best part is boating home, instead of hoofing it out with the hoards of guests.
Enjoy, GOldi
