1) Children/YA adventure book. Set on Hawaii or a very similar volcanic tropical island in the Pacific, this book features (I *think*) four young people who are friends, almost certainly 2 boys and 2 girls. The main adventure they end up on is getting lost in the rainforest on the island and running into a Japanese soldier who is still fighting WWII (I think the date of the book is sometime in the 1960s, cannot be later than the 70s). One of the kids is hurt and so they cannot easily escape at first. Finally, when they DO escape, it's in a jeep or something similar which, in the grand finale, they are driving down an empty lava tube and the volcano begins to erupt, sending lava thundering down the tube. They just barely outrace the lava flow, exiting the tube with only seconds to spare before the flow bursts out.
2) A collection of stories about people's adventures in underwater settings; my hazy memory claims the title was simply "Underwater!" with a vague image of a diver menaced by a shark. The one story I recall vividly from the book involves a man fishing in a lake (in Arkansas, as I remember) who is attacked by a huge shark; the shark hits the boat, he goes overboard, evades (barely) one pass from the shark, and then on the second pass manages to gut the thing by pushing DOWN off the boat just as it charges and then stabbing upward with his fish knife.
Again, any ideas?