This rough east-cost map (located by Jim Woosley), made it clear that I had one place in Costa Rica where I could drop my wreck and use the methods I want to use to have my folks find it. Boca del Rio Colorado, which serves as the border between Costa Rica and Nicaragua. Isla Brava, slightly inland from the mouth of the river, may play a part, too. This isn’t ideal — a place close to Limon, much farther south, would have been perfect. But the shelf didn’t cooperate.
And silt deposition from the river may actually play into my favor, story-wise.
Thanks to everyone who hunted or is hunting. Additional links are still welcome, and I’ll give credit to finders for those I can use.
Wish I could send my folks to the west coast. It’s much better designed for my needs, but that would entail a trip through the Panama Canal, which would be a plot drag.