If the heaviest thing the trailer will see is a raft, I suspect the 5/8 MDO would work okay.
I have 3/4 MDO on two boats, a 14' and 18', for side decks and drop hatch. I bought two sheets of 5/8 when i was sourcing material. The 5/8 would work but I would recommend the 3/4 unless weight was your biggest concern. The difference in weight is only a few pounds (~4lbs on the 14') but the difference in strength is very noticeable. Mostly in that the 3/4 doesn't flex and I don't worry about throwing dry boxes and coolers (including loaded Engel 240!) on it while loading the boat. If I was going to fly, portage, or pack in frequently I'd probably look at something else entirely or just run without decks.
I spent quite a bit of time filling voids, routering edges and holesawing strap holes. I then primered and painted with textured deck paint product. Only one season on them so far but pretty happy with how they turned out.