Just got my first notification on this thread in weeks. Had no idea you'd all been chatting away.
We did not purchase insurance. It's a calculated risk I guess.
My kids are 5 and 3 (leaving the 1YO with grandma), so no stroller for us.
As for excursions, we'll probably want to get the kids to the Turtle Farm on Grand Cayman (until I tell my DW what they raise the turtles for). DW is also enchanted with the submarine idea, so maybe do that in Cozumel. If it was just the two of us, we'd probably make a run at the ruins there. We mostly plan to take it easy and do low stress activities.
I'm not sure we're operating at a high enough level to be in on the fish extender thing. We'll be lucky just to get there with the essentials.