I've gotten to the sea caves and looked around. Used the sledgehammer and found the locked door. I also have a wooden plank I haven't found a use for. I think I'm supposed to find the idol here. Any help along these lines would be greatly appreciated, thanx.

Spoiler: look for a fishing line, and a hole in the roof with a beam to tie it to.

Ahhh, I have the fishing line, and I've noticed a lot of holes in the roofs of the tunnels, even fell thru some of them...

Thanx for the tip!

You will not find the idol in the sea caves. They are located somewhere else. BTW the hole that you're looking for has a stick running across the opening. That should narrow it down a bit.

Yeah, well, I finally meet Marten, after going thru all that stuff, and he won't talk to me because I don't have the Helm of Serenity, which I have never heard of before this. I don't recall any previous mention of the HoS. I guess I just ask everyone I know about it, or have I missed something?

You are not supposed to meet Marten just yet. You should go to Martens Bluff first and find the idol and words (quest from shaman in Trynton). When you return to the shaman in Trynton you will be able to get the Helm of Serenity there.
I recomend setting a portal at Marten, so you can easily return to him with the helm in your bagpack.