Well first up to need to determine why the frame has pulled away from that floor timber. More than likely rotted wood or bad fastener. Because you are loose in that area is why you have flex. You are supposed to soak yes but you need to look into some refastening at minimum and look at the structure. The bile to me from the pictures looks paint sick. In a lapstrake boat that will be no easy task cleaning up.
Check your frames at the bend in the chine area, get the floor timbers refastened or replace them then there are several other options for sistering the frames. Anytime you do put in a sister rib it needs to be fastened into the longitudinal s and the rabbit of the keel if possible.
It is ok for a wood boat to have some movement you do not want them brittle but from you posting you are getting some excessive play n the boat and before you have a problem with a boat load of people get on it. Start with the floor timbers and go from there. USE BRONZE fasteners. Get your self some wood boat restoration books so you under stand some basic rules with fasteners.
I took a closer look today and took some pictures. Also a neighbor came over today who is very knowledgeable about repairing boats and he took a look and explained some of how its put togetheir and some suggestions.
I most definately will have to get some books to help me learn about wooden lapstrake boats, the terminology and what/how to maintain them......so bear with me while I learn.
As you can see all the side boards(port to starboard) that support the chine have been lifted but in the center portion of those boards are solid and secure. And it looks like an attempt has been made to repair it at least once before. Most of the tapered ends of these boards have had too many patch jobs done on them from my observations.
I poked around with an ice pic looking tool on here and there and did not find any rot but then again I don't know what, where and how to look. There are some underside spots where it looks like the paint has cracked and separated from flexing too.
I'm told this 59 mahagony boat was made in Hong Kong by Noregian or Sweedish boat builers. Its registered and listed as Asiatice Pleasure with the DMV. There is a brand name but I'll have to get that later.
Jim and Andreas thanks for for your reply and interest. Sorry but I don't know much of the history on any repairs but I do know its a one owner family boat passed down to his son. I would believe its original wood with a poor patch job. I got a book coming called "Restore Your Wooden Boat: How to Do It by Those Who've Done It"
I'm just getting my feet wet and have much to learn and much work to be done but most likely I'll hire a more experienced person to do the work I don't want to attempt to do quite yet.