I’d do a Magic Kingdom cruise (if one is available for midnight) [B][U]IF[/U][/B] they are offering one.
That’s a big if because they launch fireworks from the lagoon as well as from the usual places. The lagoon is a mirror of the in park show.
But if they are offering midnight cruises, MK is your better bet because there is only room for six boats for Illuminations and they will all be gone by 7:01 AM the first morning you can reserve a boat.
Conversely, with 5 different marinas and a much bigger area to work with than underneath a bridge, there should be on the order of 15-20 boats total out on Seven Seas Lagoon.
But you could still give the Illuminations cruise a shot.
The Illuminations cruise ties up under the bridge between England and France, you you are inside the park.
IF they are doing midnight cruises for Fantasy in the Sky at MK, you won’t be inside the park, but fireworks will be launching all around you. Either way, you won’t have to deal with the in park crush.