Reply to W_bovine:
I'm guessing you're right. Meanwhile, the Google store has one review saying it works on Nexus 7 and one saying not. Isn't that helpful?
Also in the Google store, just looking without an account, I didn't see any distinction in Navionics apps for phones vs tablets. I asked Navionics about trying it out. Their reply was that the Google store has a 15 minute return policy.
Bottom line is that I'm guessing there's a good chance it will work. All I'd lose is $15 (I need USA + Canada) if it took me more than 15 minutes to find out it didn't.
I guess there's no rush. Maybe I'll wait until there are discounts on the Nexus 7.