I did a backpacking trip on Isle Royale last week, and took my Yak along, more to mess around on lakes than anything. While it seems that few if any of the small rivers and creeks on the island have enough volume to be packraftable, the park has huge potential for cool hiking/flatwater floating trips.
Even larger drainages like Washington Creek look too small and bony, even I imagine at high water, to make a worthwhile float. I even tried to paddle the river/slough that connects McCargoe Cove and Chickenbone Lake, but got shut down by shallow water and abundant beaver-felled trees less than 1/2 mile from the mouth. Perhaps the Siskiwit Rivers might go, but given their marshy nature on the map I wouldn’t bet on it.
What Isle Royale does have in abundance is interior lakes and exterior bays and islands. A packraft would allow for some very creative routes. For instance, the N-S canoe route through the middle of the island is heavy on portaging, and a packraft might well be a fun option. The NE sector of the island is a maze of bays, peninsulas, isthmuses, and small islands, and would provide for fun exploring and mostly protected water. Parts of the southern shore SW of Feldtmann Ridge also looked (from our flight in) to have long sections of gravel beaches, with rock headlands to float around.
Off trail hiking is certain not to be taken lightly. While some of the island is mature spruce or birch forest with a surprisingly open understory, much more of it is thick brush, cedar bog, or outright swamp. Beaver bogs are profligate. You’d probably see some moose, though.