Had a very old Otter with same problem. Patched any obvious leaks changed valves - no better. Tried inflatable marine sealant that you put inside the tubes and roll raft around and it fills in from inside. Didn’t do the floor - just put air in mid float. It is like a new boat. We have another boat now so just use it for day trips, but it holds air all day. Apparently there’s a problem with multiple pin holes with old hypalon boats.
If you lean over to watch the river slipping slowly away beneath you, you will suddenly know everything there is to be known. Winnie the Pooh