Weird... the first and fourth picture remind of The Blue Lagoon.
The part of the movie with the boy and girl playing on the beach and finding the turtle reminded me of that film as well. Not so much later, which is due to Magnus' clothing and the scenery. The shell necklace that Magnus wears probably helped you associate the other scene with "The Blue Lagoon."
Hey, a hybrid film that adds Gamera to "The Blue Lagoon" could be fun. That or "The Blue Lagoon" crossed with "Shock Waves."
How did they get their boat way up on a rock like that? They're going to have a hell of a time explaining it to the insurance company.
That's the turtle. It's upset about having a helicopter kamikaze into it and being harpooned by Carl Weathers. The scenes with the model boat vary from barely acceptable to completely silly, but the worst has to be when Carl Weathers shoots the turtle with his harpoon bazooka. They used some split screen shots with a real Carl and a stop motion turtle; that looks generally OK. For the rest of the shots they tried something else. It looks like there is a huge bubbling tarp in the ocean near the (real) ship; that doesn't look so OK.
You forgot the worst scene of all, when the 100ft demon-turtle sneaks up on the scientist in the lab. Classic.
When it eats Magnus' father! That is plain awful, and then most of the mansion slides down into the water since the scientists watery grotto of a laboratory is in a cave under his house.